refactor(subprocess): keep terminate() as the seam's only termination verb

Delete kill(signal?) from SubprocessHandle: consumers stop a process only
through terminate()'s tree-scoped SIGTERM→graceMs→SIGKILL escalation
(idempotent, also driven by the spec's abort signal, a no-op once the tree
is gone). The single-signal verb had exactly one consumer family —
lsp-local — and what it bought there was a private re-implementation of
the same escalation. The internal kill closure stays in spawn.ts as the
dispose ladder's tier primitive; terminate() now routes through it too.

lsp-local collapses onto the seam's escalation:
- LspConnection replaces its terminate()/kill() pair with one terminate()
  that delegates to handle.terminate(). Behavior change: the
  framing-failure path terminates instead of instant SIGKILL, so a
  misbehaving server now gets SIGTERM plus the killGraceMs window to
  flush before SIGKILL.
- ConnectionSpec.pipeDrainGraceMs becomes killGraceMs: one grace, the
  spawn spec's graceMs, drives both the escalation window and post-exit
  pipe draining (the provider already passed killGraceMs for it).
- LspInstance.forceTerminate() drops its hand-rolled bounded first wait
  (LSP_KILL_GRACE) and escalateProcessTree (deleted with its export and
  unit test): the seam's escalation already commits to SIGKILL after
  killGraceMs, so only the unbounded quiescence awaits stay load-bearing.

Tests: kill()-shaped spawn specs become terminate()-shaped or fold into
the terminate() suites (group-wide delivery; the settled no-op case was
already pinned by 'terminate() after the tree died'); tree-survivor
coverage is intact. A stderr-'inherit' disposition test completes the
stdout/stderr symmetry so the scoped subprocess+lsp coverage gate stands
alone instead of leaning on subagent-acp's cross-package runs.

Docs: SubprocessHandle type-equiv block, seam/impl/group READMEs, and the
consumer-migration Agent Note lose the kill(signal?) vocabulary (zh pairs
re-recorded); cordis api/services catalogs regenerated.

packages/subprocess/subprocess-local/src/spawn.ts

@@ -392,11 +392,12 @@ export function spawnSubprocess(spec: SubprocessSpawnSpec, internals: SpawnInter
     }
   }
 
-  const kill = (sig: NodeJS.Signals = 'SIGTERM'): void => {
-    // Guard on TREE liveness, not outcome settlement: a TERM-trapping helper
-    // can outlive the settled direct child and must stay signalable, while a
-    // fully-dead tree (possible pid reuse) must not be re-signalled from a
-    // caller's finally block.
+  // The dispose ladder's tier primitive (not on the handle — terminate() is
+  // the only consumer-facing termination verb). Guards on TREE liveness, not
+  // outcome settlement: a TERM-trapping helper can outlive the settled direct
+  // child and must stay signalable, while a fully-dead tree (possible pid
+  // reuse) must not be re-signalled by a later tier.
+  const kill = (sig: NodeJS.Signals): void => {
     if (!treeAlive()) return
     signalTree(platform, pid, sig, child, taskkill)
   }
@@ -404,15 +405,13 @@ export function spawnSubprocess(spec: SubprocessSpawnSpec, internals: SpawnInter
   const terminate = (): void => {
     if (graceTimer !== undefined) return // escalation already in flight
     if (!treeAlive()) return
-    signalTree(platform, pid, 'SIGTERM', child, taskkill)
+    kill('SIGTERM')
     // The escalation must survive direct-child settlement — the leader dying
     // does not mean the tree died — so settle does not clear this timer, and
-    // it re-probes tree liveness before force-killing. It stays ref'd: the
-    // pending SIGKILL is a commitment, and a parent exiting before it fires
-    // would orphan a trapped survivor. Self-bounds at graceMs.
-    graceTimer = setTimeout(() => {
-      if (treeAlive()) signalTree(platform, pid, 'SIGKILL', child, taskkill)
-    }, spec.graceMs)
+    // kill() re-probes tree liveness before force-killing. It stays ref'd:
+    // the pending SIGKILL is a commitment, and a parent exiting before it
+    // fires would orphan a trapped survivor. Self-bounds at graceMs.
+    graceTimer = setTimeout(() => { kill('SIGKILL') }, spec.graceMs)
   }
 
   // The caller owns timeout classification; this layer only reacts to abort.
@@ -514,7 +513,6 @@ export function spawnSubprocess(spec: SubprocessSpawnSpec, internals: SpawnInter
       ...stderrCollector !== undefined ? { stderr: stderrCollector } : {},
     },
     done,
-    kill,
     terminate,
     waitForExit,
     dispose,