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).

packages/subprocess/README.md

@@ -0,0 +1,10 @@
+# subprocess/ — subprocess capability family
+
+The shared home for spawning managed child-process groups: fully-specified spawn specs, bounded tail-keep output with spill files, credential-scrubbed environments, offset-based incremental reads, and SIGTERM→grace→SIGKILL group kills. Command defaulting, shell semantics, deadlines, and presentation stay with consumers — the [bash executor family](../bash/README.md) is the first and owning consumer. See the [subprocess seam Agent Note](../../.agents/notes/implemented/architecture/2026-07-26-subprocess-seam.md).
+
+| Package | ctx key | Role |
+|---|---|---|
+| [`subprocess`](subprocess/README.md) (`@deepseek-ai/dsh-subprocess`) | `ctx.subprocess` | The seam: abstract `SubprocessService.spawn(spec)`, the fully-explicit `SubprocessSpawnSpec`, `SubprocessHandle` with offset-based readers, and the shared `DSH_*` managed-environment and `CollectedOutput` vocabulary |
+| [`subprocess-local`](subprocess-local/README.md) (`@deepseek-ai/dsh-subprocess-local`) | — | The local implementation: detached process groups, tail-keep truncation with bounded private spill files, the credential scrub and `DSH_*` merge order, kill escalation, and kill-and-join disposal |
+
+The service owns process lifetime across consumer reloads; consumers own what a process means (a bash command, a future non-shell runner) and every default that shapes one.