fix(acpx): move managed installs into plugin state dir

[shirtlessfounder]

Summary

This fixes the older ACPX bootstrap path in activeclaw so default ACPX repair/install no longer targets the installed extension directory.

Root cause on activeclaw/main:

• extensions/acpx/src/service.ts never threaded the plugin service stateDir into ACPX config resolution.
• extensions/acpx/src/config.ts defaulted the unmanaged ACPX command to the extension-local binary path.
• extensions/acpx/src/ensure.ts repaired ACPX by running npm install in that install-root path, which is brittle for packaged/read-only installs and matches the roadblock evidence.

What changed:

• add stateDir support to ACPX config resolution
• default the managed ACPX command/install root to <plugin-state>/acpx/...
• pass ctx.stateDir from the plugin service into ACPX config
• repair ACPX into the managed writable install root instead of the extension directory
• update ACPX router/docs text so repo-local repair no longer tells agents to write directly into extensions/acpx
• add regression coverage for config, service, ensure, and runtime interactions

Verification

Ran:

corepack pnpm vitest run \
  extensions/acpx/src/config.test.ts \
  extensions/acpx/src/ensure.test.ts \
  extensions/acpx/src/runtime.test.ts \
  extensions/acpx/src/service.test.ts

Result:

• 4 test files passed
• 45 tests passed

Also ran:

corepack pnpm exec oxfmt --check \
  extensions/acpx/src/config.ts \
  extensions/acpx/src/service.ts \
  extensions/acpx/src/ensure.ts \
  extensions/acpx/src/config.test.ts \
  extensions/acpx/src/ensure.test.ts \
  extensions/acpx/src/service.test.ts \
  extensions/acpx/openclaw.plugin.json \
  extensions/acpx/skills/acp-router/SKILL.md \
  docs/tools/acp-agents.md