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).

examples/headless-agent/composition.md

@@ -10,8 +10,8 @@ flowchart LR
   cfg["examples/headless-agent<br/>cordis.yml"]
   plugin_headless_llm_deepseek["llm-deepseek<br/>@deepseek-ai/dsh-llm-deepseek"]
   cfg --> plugin_headless_llm_deepseek
-  plugin_headless_processes["processes<br/>@deepseek-ai/dsh-process-local"]
-  cfg --> plugin_headless_processes
+  plugin_headless_subprocess["subprocess<br/>@deepseek-ai/dsh-subprocess-local"]
+  cfg --> plugin_headless_subprocess
   plugin_headless_bash["bash<br/>@deepseek-ai/dsh-bash-local"]
   cfg --> plugin_headless_bash
   plugin_headless_cli_agent["cli-agent<br/>@deepseek-ai/dsh-cli-demo"]
@@ -56,7 +56,7 @@ flowchart LR
 | Plugin id | Package / module |
 | --- | --- |
 | `llm-deepseek` | `@deepseek-ai/dsh-llm-deepseek` |
-| `processes` | `@deepseek-ai/dsh-process-local` |
+| `subprocess` | `@deepseek-ai/dsh-subprocess-local` |
 | `bash` | `@deepseek-ai/dsh-bash-local` |
 | `cli-agent` | `@deepseek-ai/dsh-cli-demo` |
 | `token-meter` | `@deepseek-ai/dsh-token-meter` |