feat: add persistent PTY sessions

docs/core-data-structures/core.md

@@ -26,6 +26,7 @@ Everything else is documented on a **sub-page**, not here. The rule that draws t
 | [user-interaction.md](user-interaction.md) | the UI-backed human question/answer seam: `AskUserQuestionRequest`, answer/options vocabulary, provider API, error taxonomy |
 | [approval.md](approval.md) | the one-shot user-approval seam: `ApprovalRequest`, `ApprovalOutcome`, per-session policy, audit and answerer contracts |
 | [bash.md](bash.md) | the bash executor seam: `BashExecRequest`/`Spec`, `BashRunResult`, background `BashProcess` handles |
+| [pty.md](pty.md) | persistent terminal ids, backend/session contracts, send readiness, bounded reads, and owner-visible snapshots |
 | [sandbox.md](sandbox.md) | the process-confinement seam: file-effect modes, `SandboxPolicy`, `ConfinedArgv`, enforcement and fail-closed errors |
 | [code-runtime.md](code-runtime.md) | the code-execution seam: `CodeRunRequest`/`Result`, binding namespaces, captured logs, the `CodeRunFailure` taxonomy |
 | [filesystem.md](filesystem.md) | the filesystem seam: `FsTarget`, read/write/edit outcomes, observed-file state, `FsErrorCode` |

docs/core-data-structures/pty.md

@@ -0,0 +1,89 @@
+# Persistent PTY Sessions
+
+Types shared by PTY backends, `ctx.pty`, and the model-facing consumer. The [persistent PTY Agent Note](../../.agents/notes/implemented/feature/2026-07-16-persistent-pty-sessions.md) owns the rationale; this page records the cross-package vocabulary from [`packages/pty/pty/src/types.ts`](../../packages/pty/pty/src/types.ts).
+
+## Identity and readiness
+
+`PtySessionId` is a service-minted branded id. Optional names are owner-local display metadata; authorization compares the exact owning `Agent`, not a name or guessed id.
+
+`PtyWaitReason` says why one send returned. It is independent from `PtySessionStatus`: silence or timeout may return while the top-level shell remains alive, while `session_exit` means that shell exited rather than an arbitrary foreground child.
+
+```ts type-equiv
+/** Why one interactive send returned control to its caller. */
+type PtyWaitReason = 'stdin_read' | 'inferred_idle' | 'timeout' | 'session_exit'
+```
+
+```ts type-equiv
+/** Top-level PTY process status, independent of a send's wait reason. */
+type PtySessionStatus =
+  | { kind: 'running' }
+  | { kind: 'exited'; exitCode: number | null; signal: NodeJS.Signals | null }
+```
+
+## Backend and live session
+
+A backend owns how one registered type starts and detects readiness. `PtyService` publishes the returned session only after setup succeeds, then owns id authorization and cleanup. A backend session owns terminal state and captured-resource quiescence.
+
+```ts type-equiv
+/** Replaceable provider for one PTY session type. */
+interface PtyBackend {
+  /** Stable type selected by {@link PtySpawnRequest.type}. */
+  readonly type: string
+  /** Create an unpublished session or reject after cleaning partial resources. */
+  spawn(spec: PtyBackendSpawnSpec): Promise<PtyBackendSession>
+}
+```
+
+```ts type-equiv
+/** Backend-owned live session retained by {@link PtyService}. */
+interface PtyBackendSession {
+  /** Initial bounded terminal output returned from `pty_spawn`. */
+  readonly motd: string
+  /** Top-level process id when one exists. */
+  readonly pid?: number
+  /** Start one exclusive send operation. */
+  startSend(request: PtySendRequest): PtySendOperation
+  /** Read one bounded page from retained scrollback. */
+  read(request: PtyReadRequest): PtyReadResult
+  /** Signal the verified foreground process group. */
+  signal(signal: PtySignal): Promise<PtySignalResult>
+  /** Observe top-level process status. */
+  status(): PtySessionStatus
+  /** Idempotently close the captured owned process tree and await quiescence. */
+  close(reason: string): Promise<void>
+}
+```
+
+## Send and retained output
+
+One live session accepts one active send. Its operation exposes a consuming output cursor for generic background tasks and one terminal result for a foreground caller. `PtyReadResult` separately pages the bounded session scrollback.
+
+```ts type-equiv
+/** Live backend-owned send; exactly one may be active per PTY session. */
+interface PtySendOperation {
+  /** Resolves after readiness, timeout, cancellation, or top-level process exit. */
+  done: Promise<PtySendResult>
+  /** Consume output produced since the prior call. */
+  readOutput(): PtySendRead
+  /** Request `SIGINT`; returns false after the operation settled. */
+  cancel(): boolean
+}
+```
+
+```ts type-equiv
+/** Settled result for one foreground or background send. */
+interface PtySendResult {
+  /** Bounded rendered terminal delta remaining at settlement. */
+  viewport: string
+  /** Why the wait returned; this does not imply arbitrary child-process exit. */
+  waitReason: PtyWaitReason
+  /** Top-level session status observed at settlement. */
+  sessionStatus: PtySessionStatus
+  /** Whether output was dropped from the operation or retained scrollback. */
+  truncated: boolean
+}
+```
+
+## Ownership and durability
+
+`PtyService` attaches one awaited cleanup to the exact owner scope, rejects foreign operations, and keeps sessions alive across backend or tool-plugin reload. PTY state and raw bytes remain process-local. Model input and bounded returned output are durable through the existing `tool/call`, `tool/result`, and task-result paths rather than duplicate PTY session events.