feat(code-mode): add ACP client engine (Layer 2 core)

Own the Agent Client Protocol client instead of shelling out to `acpx`, so code
mode can stream structured events (tool calls, diffs, plan) and surface live
permission requests. Headless acpx can't do live approvals (it only supports
--approve-all), which is why we drive the agent adapters ourselves.

- code-mode/acp/{agents,client,permission-broker,session-store,manager,types}.ts:
  headless engine driving the Claude/Codex ACP adapters; one warm session per chat
  with create-or-resume via session/load; approval policy (ask | auto-approve-reads
  | yolo) in the broker.
- claude-exec.ts: cross-platform claude resolver (Windows .cmd EINVAL fix + macOS/Linux
  GUI-PATH safety net) shared with the legacy acpx path in builtin-tools.ts.
- add @agentclientprotocol/sdk + claude/codex adapters to core.

apps/x/packages/core/package.json

@@ -11,6 +11,9 @@
     "test:watch": "vitest"
   },
   "dependencies": {
+    "@agentclientprotocol/claude-agent-acp": "^0.39.0",
+    "@agentclientprotocol/codex-acp": "^0.0.44",
+    "@agentclientprotocol/sdk": "^0.22.1",
     "@ai-sdk/anthropic": "^2.0.63",
     "@ai-sdk/google": "^2.0.53",
     "@ai-sdk/openai": "^2.0.91",