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/src/index.ts

@@ -0,0 +1,62 @@
+/**
+ * The subprocess seam (`ctx.subprocess`): spawn fully-specified
+ * commands into managed process groups with bounded, spill-backed output and
+ * escalated kills. Command defaulting, shell semantics, deadlines, and
+ * presentation belong to consumers — the bash executor seam is the owning
+ * template. The local implementation lives in
+ * `@deepseek-ai/dsh-subprocess-local`.
+ * @module @deepseek-ai/dsh-subprocess
+ */
+
+import { Context, Service } from 'cordis'
+import type { SubprocessHandle, SubprocessSpawnSpec } from './types.ts'
+
+export { DSH_ENV_PREFIX } from './types.ts'
+export type {
+  CollectedOutput,
+  DshEnvironment,
+  DshEnvironmentKey,
+  SubprocessHandle,
+  SubprocessOutcome,
+  SubprocessOutputRead,
+  SubprocessOutputReader,
+  SubprocessSpawnSpec,
+} from './types.ts'
+
+declare module 'cordis' {
+  interface Context {
+    subprocess: SubprocessService
+  }
+}
+
+/**
+ * Abstract subprocess service. Subclass, implement {@link spawn}, and load the
+ * subclass as a plugin — it registers as `ctx.subprocess` (one implementation
+ * per context; loading a second throws, which is cordis' standard
+ * duplicate-service behavior).
+ *
+ * Implementations must honor these semantics:
+ * - {@link spawn} returns immediately with a live handle; `done` resolves at
+ *   process close and rejects only for spawn-level failures.
+ * - Output readers are offset-based and non-consuming, so independent readers
+ *   never consume one another's output; lossy reads report truncation and the
+ *   spill file holding the complete stream when one exists.
+ * - {@link SubprocessHandle.kill} and the spec's abort signal escalate
+ *   SIGTERM→grace→SIGKILL across the whole process group.
+ * - Disposal kills all still-running managed processes and awaits their exit.
+ */
+export abstract class SubprocessService extends Service {
+  constructor(ctx: Context) {
+    super(ctx, 'subprocess')
+  }
+
+  /**
+   * Start one managed child process from a fully-specified spec; this seam
+   * applies no defaults.
+   * @param spec - argv, directory, limits, grace, cancellation, and environment.
+   * @returns the live process handle (readers, kill, outcome promise).
+   */
+  abstract spawn(spec: SubprocessSpawnSpec): SubprocessHandle
+}
+
+export default SubprocessService