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/bash/bash/src/index.ts

@@ -44,8 +44,8 @@ declare module 'cordis' {
  * - {@link BashProcess.readOutput} is incremental: consecutive reads never
  *   repeat output. Lossy reads report truncation and available spill files.
  * - A still-running background process is stopped and awaited when its
- *   owning composition tears down. With the process-manager seam that
- *   boundary is `ctx.processes` disposal, so a background process survives
+ *   owning composition tears down. With the subprocess seam that
+ *   boundary is `ctx.subprocess` disposal, so a background process survives
  *   an executor-only reload.
  */
 export abstract class BashExecutor extends Service {