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/acp-agent/composition.md

@@ -14,8 +14,8 @@ flowchart LR
   cfg --> plugin_acp_sandbox
   plugin_acp_sandbox_policy["sandbox-policy<br/>@deepseek-ai/dsh-sandbox-policy"]
   cfg --> plugin_acp_sandbox_policy
-  plugin_acp_processes["processes<br/>@deepseek-ai/dsh-process-local"]
-  cfg --> plugin_acp_processes
+  plugin_acp_subprocess["subprocess<br/>@deepseek-ai/dsh-subprocess-local"]
+  cfg --> plugin_acp_subprocess
   plugin_acp_bash["bash<br/>@deepseek-ai/dsh-bash-sandbox"]
   cfg --> plugin_acp_bash
   plugin_acp_approval["approval<br/>@deepseek-ai/dsh-user-approval"]
@@ -70,7 +70,7 @@ flowchart LR
 | `llm-deepseek` | `@deepseek-ai/dsh-llm-deepseek` |
 | `sandbox` | `@deepseek-ai/dsh-sandbox-local` |
 | `sandbox-policy` | `@deepseek-ai/dsh-sandbox-policy` |
-| `processes` | `@deepseek-ai/dsh-process-local` |
+| `subprocess` | `@deepseek-ai/dsh-subprocess-local` |
 | `bash` | `@deepseek-ai/dsh-bash-sandbox` |
 | `approval` | `@deepseek-ai/dsh-user-approval` |
 | `acp-agent` | `@deepseek-ai/dsh-acp-demo` |