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/README.md

@@ -0,0 +1,26 @@
+# @deepseek-ai/dsh-subprocess
+
+The subprocess seam (`ctx.subprocess`). The abstract `SubprocessService` exposes one method — `spawn(spec): SubprocessHandle` — plus the vocabulary shared by every consumer: the fully-explicit `SubprocessSpawnSpec`, `SubprocessHandle` with its non-consuming offset-based output readers, `SubprocessOutcome`, `CollectedOutput`, and the managed `DSH_*` environment namespace (`DSH_ENV_PREFIX`, `DshEnvironment`). The local implementation lives in [`dsh-subprocess-local`](../subprocess-local/README.md).
+
+## Contract
+
+- `spawn(spec)` returns immediately with a live handle; `done` resolves at process close and rejects only for spawn-level failures.
+- The spec is fully explicit — argv, cwd, per-stream byte caps, spill cap, grace — because deployment-varying defaults belong to the calling seam's config, not to a hidden subprocess-service default (the `dsh-bash` request/spec split is the owning template). `argv` is never shell-interpreted here; a consumer that wants a shell passes `['bash', '-c', command]` itself.
+- Output readers take whole-stream byte offsets and never consume: independent readers cannot steal one another's deltas. A read whose offset slid out of the in-memory tail is `lossy` and points at the full-stream spill file when one exists.
+- `kill()` and the spec's abort signal escalate SIGTERM→grace→SIGKILL across the whole detached group; the service reacts to the abort but never classifies why (callers own deadlines and cause classification).
+- Disposal kills all still-running managed processes and awaits their exit.
+
+See the [process data-structure catalog](../../../docs/core-data-structures/subprocess.md) and the [seam Agent Note](../../../.agents/notes/implemented/architecture/2026-07-26-subprocess-seam.md).
+
+## Model Experience
+
+Indirectly, through consumer seams (today the bash executor family behind `dsh-tool-bash`), which own all model-facing rendering of process output and lifecycle.
+
+#### KV Cache effect
+
+No direct invalidation; the named consumers own any request-prefix changes.
+
+## Known Limitations and Deferred Work
+
+- **One consumer family so far** — the seam's shape is proven against the bash executors only; the other in-repo spawn sites (LSP servers, PTY backends, subagent transports) keep their own bespoke process handling until their stream/lifecycle needs are re-examined against this contract.
+- **POSIX group semantics are assumed** — the handle vocabulary (`pid` as group leader, group kills, SIGTERM/SIGKILL escalation) has no Windows story.