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).

scripts/gen-doc-graphs.ts

@@ -266,13 +266,13 @@ const SERVICE_ROLES: ServiceRole[] = [
     note: 'Folds revisioned objective state from the session log and keeps live continuation activation process-local.',
   },
   {
-    key: 'processes',
+    key: 'subprocess',
     pkg: 'process',
     title: 'Process manager seam',
     mode: 'seam',
     implementations: ['process-local'],
     consumers: ['bash-local', 'bash-sandbox'],
-    note: 'The bash executors spawn their process groups through ctx.processes; the manager owns group lifetime, bounded spill-backed output, and kill escalation.',
+    note: 'The bash executors spawn their process groups through ctx.subprocess; the service owns group lifetime, bounded spill-backed output, and kill escalation.',
   },
   {
     key: 'bash',