refactor(subprocess): rename the process seam to subprocess and address review

Review feedback (tianyicui): 'process' is a poor service name. The family is
now packages/subprocess/ — @deepseek-ai/dsh-subprocess (ctx.subprocess,
abstract SubprocessService, Subprocess* vocabulary) and
@deepseek-ai/dsh-subprocess-local (LocalSubprocessService) — renamed
throughout code, compositions, docs (en+zh, pairs re-recorded), catalogs,
and gates. 'subprocess' is the precise term for managed OS children (the
Python-stdlib sense), avoids colliding with Node's global process object,
and reads as one system beside dsh-subagent-subprocess.

ds-review-bot findings addressed:
- kill() on a settled handle is now a no-op (no signal to a possibly-reused
  pgid, no referenced grace timer delaying exit); pinned by a spy test.
- The moved DshEnvironmentKey/DshEnvironment/CollectedOutput types get
  drift-checked type-equiv blocks on the new subprocess.md page, restoring
  their manifest registration.
- subprocess.md is registered in the core.md sub-page index (en+zh).

docs/core-data-structures/subprocess.md

@@ -0,0 +1,159 @@
+# Subprocess
+
+The subprocess seam is split across interface ([dsh-subprocess](../../packages/subprocess/subprocess), `ctx.subprocess`) and implementation ([dsh-subprocess-local](../../packages/subprocess/subprocess-local)); its consumers are other capability seams — today the [bash executor family](bash.md), which passes `['bash', '-c', command]` argv and owns every default. This seam owns the managed `DSH_*` environment namespace and the `CollectedOutput` shape; [dsh-bash](../../packages/bash/bash) re-exports them so bash consumers keep one import root.
+
+Source: [`packages/subprocess/subprocess/src/types.ts`](../../packages/subprocess/subprocess/src/types.ts)
+
+## Managed environment namespace and captured output
+
+`DSH_*` variables are Harness-owned child-process facts; implementations discard ambient `DSH_*` names before merging the caller's snapshot, and each captured stream reports its truncation and spill-recovery state through `CollectedOutput`.
+
+```ts type-equiv
+/** One environment key inside the managed {@link DSH_ENV_PREFIX} namespace. */
+type DshEnvironmentKey = `${typeof DSH_ENV_PREFIX}${string}`
+```
+
+```ts type-equiv
+/** Trusted DeepSeek Harness variables for one child-process execution. */
+type DshEnvironment = Readonly<Record<DshEnvironmentKey, string>>
+```
+
+```ts type-equiv
+/** One captured stream: the (possibly truncated) text plus recovery info. */
+interface CollectedOutput {
+  /** Collected text — the TAIL of the stream when truncated. */
+  text: string
+  /** True when bytes were dropped from `text`. */
+  truncated: boolean
+  /** Path to a file holding the COMPLETE stream, when truncated and available. */
+  spillPath?: string
+}
+```
+
+## The fully-explicit spawn spec
+
+The seam applies no defaults: every limit and directory is explicit on the spec, so the caller's own config — not a hidden subprocess-service default — decides them. `argv` is never shell-interpreted.
+
+```ts type-equiv
+/**
+ * A fully-specified spawn request. This seam applies no defaults: every limit
+ * and directory is explicit, so the caller's own config — not a hidden
+ * subprocess-service default — decides them (the `dsh-bash` request/spec split
+ * is the owning template).
+ */
+interface SubprocessSpawnSpec {
+  /** Executable and arguments; `argv[0]` is the program. Never shell-interpreted here. */
+  argv: readonly string[]
+  /** Working directory for the child. */
+  cwd: string
+  /** Stdout in-memory cap; overflow spills to disk (tail kept in memory). */
+  stdoutMaxBytes: number
+  /** Stderr in-memory cap; overflow spills to disk (tail kept in memory). */
+  stderrMaxBytes: number
+  /** Per-stream spill-file cap; larger streams retain only their in-memory tail. */
+  maxSpillBytes: number
+  /** Grace period for kill escalation and for inherited pipes after process exit. */
+  graceMs: number
+  /**
+   * Abort signal — kills the process group when it fires. The caller owns
+   * deadlines and cause classification; this seam only reacts to the abort.
+   */
+  signal?: AbortSignal | undefined
+  /**
+   * Bytes to write to the child's stdin, then close it. Absent (or empty)
+   * leaves stdin closed/empty.
+   */
+  stdin?: string | undefined
+  /**
+   * Ordinary environment entries merged after the implementation's credential
+   * scrub. `DSH_*` names are rejected and belong in {@link dshEnv}.
+   */
+  env?: Record<string, string> | undefined
+  /**
+   * Harness-owned `DSH_*` variables for this execution. Implementations
+   * discard ambient `DSH_*` entries before merging this snapshot, so an
+   * unavailable current fact cannot inherit a stale value from the harness
+   * process, and reject non-`DSH_*` names supplied through this channel.
+   */
+  dshEnv?: DshEnvironment | undefined
+}
+```
+
+## Handles and offset-based reads
+
+A spawn returns a live handle immediately. Output readers take whole-stream byte offsets and never consume, so independent readers cannot steal one another's deltas; the consuming-cursor model the bash tool presents is consumer-owned state over these readers.
+
+```ts type-equiv
+/**
+ * A live child process. `kill()` starts the group SIGTERM→grace→SIGKILL
+ * escalation; buffered output remains readable after exit.
+ */
+interface SubprocessHandle {
+  /** Process id (group leader); -1 when the spawn itself failed. */
+  readonly pid: number
+  /** Live stdout reader (also readable after exit). */
+  readonly stdout: SubprocessOutputReader
+  /** Live stderr reader (also readable after exit). */
+  readonly stderr: SubprocessOutputReader
+  /** Resolves when the process closes; rejects only for spawn-level failures. */
+  readonly done: Promise<SubprocessOutcome>
+  /** Begin SIGTERM→grace→SIGKILL on the process group. Idempotent. */
+  kill(): void
+}
+```
+
+```ts type-equiv
+/**
+ * Cursor-free incremental access to one live output stream. Offsets are
+ * whole-stream byte coordinates owned by the caller, so independent readers
+ * cannot consume one another's output.
+ */
+interface SubprocessOutputReader {
+  /**
+   * Read everything captured since `fromByte`. When that offset has slid out
+   * of the in-memory tail window the read is `lossy` — it returns the whole
+   * retained tail and the gap is only recoverable from the spill file.
+   * @param fromByte - whole-stream offset to resume from (a prior read's `nextOffset`; 0 for the first read).
+   * @returns the delta text, the next offset, the `lossy` flag, and the spill path when one exists.
+   */
+  readFrom(fromByte: number): SubprocessOutputRead
+}
+```
+
+```ts type-equiv
+/** One incremental {@link SubprocessOutputReader.readFrom} read. */
+interface SubprocessOutputRead {
+  /** Stream text from the requested offset (the whole retained tail when lossy). */
+  text: string
+  /** Whole-stream offset to resume from on the next read. */
+  nextOffset: number
+  /** True when the requested offset slid out of the in-memory tail window. */
+  lossy: boolean
+  /** Path to the full-stream spill file, when one was created and remains intact. */
+  spillPath?: string
+}
+```
+
+## Outcomes carry no cause classification
+
+`done` reports raw exit facts. The service kills on abort but never decides why — the caller reads the deadline signal it owns to classify timeout versus cancellation (the bash executor's `timedOut`/`aborted` split).
+
+```ts type-equiv
+/**
+ * Raw outcome of one closed process. Deliberately carries NO timeout or
+ * cancellation classification: the service kills on abort but does not decide
+ * why — the caller reads the signal it owns to classify causes.
+ */
+interface SubprocessOutcome {
+  /** Exit code; null when the process died from a signal. */
+  exitCode: number | null
+  /** Terminating signal (e.g. 'SIGTERM'); null on normal exit. */
+  signal: NodeJS.Signals | null
+  stdout: CollectedOutput
+  stderr: CollectedOutput
+}
+```
+
+## Service behavior
+
+The abstract [`SubprocessService`](../../packages/subprocess/subprocess/src/index.ts) seam defines `spawn` only; [`LocalSubprocessService`](../../packages/subprocess/subprocess-local/src/index.ts) is the local implementation (detached groups, tail-keep spill-backed collection, credential scrub, kill-and-join disposal). See [`dsh-subprocess`](../../packages/subprocess/subprocess/README.md) for the seam contract and [`dsh-subprocess-local`](../../packages/subprocess/subprocess-local/README.md) for the mechanics.