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/subagent/subagent-subprocess/src/invariant.ts

@@ -10,7 +10,7 @@ import type { InvariantInstaller } from '@deepseek-ai/dsh-invariants'
 const PACKAGE_NAME = '@deepseek-ai/dsh-subagent-subprocess'
 
 /** Cordis companion plugin name. */
-export const name = 'subagent-subprocess-invariant'
+export const name = 'subagent-subsubprocess-invariant'
 /** Service required before the companion can reserve package ownership. */
 export const inject = ['invariants']