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/subprocess/subprocess-local/src/index.ts

@@ -0,0 +1,53 @@
+/**
+ * Local-subprocess implementation of the subprocess seam. Each spawn is
+ * a detached process group with bounded, spill-backed output; disposal kills
+ * and joins live groups. It has no config: every limit arrives on the spec,
+ * so the deployment-varying choices stay with the calling seam's config (the
+ * bash executor's, today).
+ * @module @deepseek-ai/dsh-subprocess-local
+ */
+
+import { Context } from 'cordis'
+import { SubprocessService } from '@deepseek-ai/dsh-subprocess'
+import type { SubprocessHandle, SubprocessSpawnSpec } from '@deepseek-ai/dsh-subprocess'
+import { spawnProcess } from './spawn.ts'
+import type { SpawnInternals } from './spawn.ts'
+
+/**
+ * Local subprocess service: detached process groups, tail-keep truncation with
+ * bounded spill files, credential-scrubbed environment, and group
+ * SIGTERM→grace→SIGKILL escalation.
+ */
+export class LocalSubprocessService extends SubprocessService {
+  /** Live handles retained only so disposal can kill and join them. */
+  private live = new Set<SubprocessHandle>()
+  /** Test seam: spill knobs forwarded to spawnProcess. */
+  internals: SpawnInternals = {}
+
+  constructor(ctx: Context) {
+    super(ctx)
+    ctx.effect(() => async () => {
+      // Await closure so even a TERM-trapping child cannot outlive the fiber.
+      const pending: Promise<unknown>[] = []
+      for (const handle of this.live) {
+        handle.kill()
+        // Spawn-failure rejections already settled and left the live set.
+        pending.push(handle.done.catch(() => {}))
+      }
+      this.live.clear()
+      await Promise.all(pending)
+    }, 'local subprocess teardown')
+  }
+
+  spawn(spec: SubprocessSpawnSpec): SubprocessHandle {
+    const handle = spawnProcess(spec, this.internals)
+    this.live.add(handle)
+    handle.done.then(
+      () => { this.live.delete(handle) },
+      () => { this.live.delete(handle) },
+    )
+    return handle
+  }
+}
+
+export default LocalSubprocessService