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/tests/service.spec.ts

@@ -21,7 +21,6 @@ class StubSubprocessService extends SubprocessService {
       stderr: undefined,
       collected,
       done: Promise.resolve({ exitCode: 0, signal: null }),
-      kill: () => {},
       terminate: () => {},
       waitForExit: () => Promise.resolve(true),
       dispose: (_graces: SubprocessDisposeGraces) => Promise.resolve(),
@@ -41,7 +40,6 @@ describe('SubprocessService seam', () => {
     })
     expect(handle.pid).toBe(1)
     expect(handle.collected.stdout!.readFrom(0)).toEqual({ text: '', nextOffset: 0, lossy: false })
-    handle.kill()
     handle.terminate()
     await expect(handle.waitForExit()).resolves.toBe(true)
     await expect(handle.dispose({ eofGraceMs: 1, graceMs: 1 })).resolves.toBeUndefined()