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/tests/service.spec.ts

@@ -0,0 +1,55 @@
+import { describe, expect, it } from 'vitest'
+import { Context } from 'cordis'
+import { SubprocessService } from '@deepseek-ai/dsh-subprocess'
+import type { SubprocessHandle, SubprocessOutputRead, SubprocessSpawnSpec } from '@deepseek-ai/dsh-subprocess'
+
+/**
+ * Minimal concrete service: a hand-built handle. The seam is spawn-only —
+ * defaulting, shell semantics, and deadlines belong to callers — so this stub
+ * is all an implementation owes the abstract class.
+ */
+class StubSubprocessService extends SubprocessService {
+  spawn(spec: SubprocessSpawnSpec): SubprocessHandle {
+    const read: SubprocessOutputRead = { text: '', nextOffset: 0, lossy: false }
+    let killed = false
+    return {
+      pid: spec.argv.length,
+      stdout: { readFrom: () => read },
+      stderr: { readFrom: () => read },
+      done: Promise.resolve({
+        exitCode: killed ? null : 0,
+        signal: null,
+        stdout: { text: 'ok', truncated: false },
+        stderr: { text: '', truncated: false },
+      }),
+      kill: () => { killed = true },
+    }
+  }
+}
+
+describe('SubprocessService seam', () => {
+  it('a concrete subclass registers as ctx.subprocess and serves the abstract API', async () => {
+    const ctx = new Context()
+    await ctx.plugin(StubSubprocessService)
+    const handle = ctx.subprocess.spawn({
+      argv: ['true'],
+      cwd: '/stub',
+      stdoutMaxBytes: 1,
+      stderrMaxBytes: 1,
+      maxSpillBytes: 1,
+      graceMs: 1,
+    })
+    expect(handle.pid).toBe(1)
+    expect(handle.stdout.readFrom(0)).toEqual({ text: '', nextOffset: 0, lossy: false })
+    handle.kill()
+    const outcome = await handle.done
+    expect(outcome.stdout.text).toBe('ok')
+  })
+
+  it('loading a second implementation throws (one processes service per context — cordis standard)', async () => {
+    const ctx = new Context()
+    await ctx.plugin(StubSubprocessService)
+    class SecondManager extends StubSubprocessService {}
+    await expect(ctx.plugin(SecondManager)).rejects.toThrow(/service "subprocess" has been registered/)
+  })
+})