Add the ACP subagent backend: out-of-process delegation (PR3)

The first OUT-OF-PROCESS subagent backend, proving the seam generalizes past the
in-process backends. @deepseek-ai/dsh-subagent-acp runs each child agent in a
spawned subprocess, driven over the Agent Client Protocol as the CLIENT — the
direction-inverted twin of the dsh-acp server bridge. Point the configured
command at the acp-agent example and the harness talks to its own process.

- Fresh process per run: start spawns, runs one ACP session (initialize →
  newSession → prompt), dispose kills the subprocess and awaits its exit.
- Minimal client stub: advertises no fs/terminal; accumulates agent_message_chunk
  text as the result output; auto-answers session/request_permission by a
  configured policy (reject default / allow). No start-time capabilities (an
  out-of-process child can't enforce the parent's depth/tool-filter); ignores
  request.parent; injects only `subagents`.
- StopReason mapping (end_turn→completed, cancelled→aborted, …); result resolves
  error/aborted on a child failure, never rejects (seam contract).
- Security: credential-shaped ambient env vars are scrubbed; the child's own key
  is forwarded only via explicit config.env. A spawn-level error (ENOENT) is
  captured and raced against the ACP drive so a bad command settles error rather
  than crashing the parent.

Testing designed at every tier: keyless integration drives a scripted mock ACP
server subprocess (cancellation incl. the pre-newSession race and a
torn-pipe-after-cancel, permission auto-answer, non-message updates, spawn
failure, HMR, export shape) at 100% coverage; a with-key e2e drives the REAL
acp-agent example process (PONG + real file write, verified on disk) — the
harness driving itself. Snapshot coverage of an ACP child is deferred as
TODO(acp-subagent-replay) (each child is its own process with its own replay).

Stayed on @agentclientprotocol/sdk 0.25.1: the proposed 0.28.x bump only
deprecates the stable ClientSideConnection/AgentSideConnection API this layer
uses (33 sites incl. the server bridge), turning no-deprecated red across code
this PR shouldn't rewrite — that fluent-API migration is its own follow-up. The
backend needs nothing 0.28.x adds.

This completes the subagent seam stack (PR1 interface → PR2 in-process → PR2.5
snapshot infra → PR3 ACP); the seam RFC moves to implemented/, amended.

docs/architecture.md

@@ -116,7 +116,7 @@ Tool schemas are deliberately **part of the assembly**: "what the model is told
 - `whenIdle()` — resolves once the agent reaches quiescence after settling out of `running` (resolves immediately when already idle; awaits the loop exit when disposed). A non-owner's quiescence-observation hook: it lets a consumer await the current work settling **without** disposing the agent. It is NOT teardown — it does not stop queued work, unregister the agent, or detach the session; a lifecycle owner tears an agent down with `await AgentHandle.dispose()` (which stops the loop, awaits its exit, and unregisters).
 - `session`, `status`, `options`
 
-**Subagents**: `spawn`/`fork` are realized by the [`@deepseek-ai/dsh-subagent`](../packages/subagent/subagent) seam (a named-provider registry on `ctx.subagents`), not a method on `Agent`. The in-process backends create the child via `ctx.agents.create` — fork seeds the child Session with a balanced completed-turn prefix of the parent's log (`CreateAgentOptions.seed`), spawn starts fresh; children are ordinary `Agent` handles so `steer()` and event subscription work uniformly. Out-of-process transports (ACP, and later A2A / Codex app-server / Claude Code SDK) register as sibling providers. See [docs/core-data-structures/subagent.md](core-data-structures/subagent.md) and [the subagent RFC](rfc/proposed/feature/2026-06-21-subagent-capability-seam.md). Inter-agent channels beyond delegation remain deferred.
+**Subagents**: `spawn`/`fork` are realized by the [`@deepseek-ai/dsh-subagent`](../packages/subagent/subagent) seam (a named-provider registry on `ctx.subagents`), not a method on `Agent`. The in-process backends create the child via `ctx.agents.create` — fork seeds the child Session with a balanced completed-turn prefix of the parent's log (`CreateAgentOptions.seed`), spawn starts fresh; children are ordinary `Agent` handles so `steer()` and event subscription work uniformly. Out-of-process transports (ACP, and later A2A / Codex app-server / Claude Code SDK) register as sibling providers. See [docs/core-data-structures/subagent.md](core-data-structures/subagent.md) and [the subagent RFC](rfc/implemented/feature/2026-06-21-subagent-capability-seam.md). Inter-agent channels beyond delegation remain deferred.
 
 ### Loop lifecycle (session / turn / step)