feat: add qwen built-in ACP agent

[gandli]

Summary

• add to the built-in agent registry
• map to the native ACP command
• extend agent-registry tests to cover the new built-in agent
• update README built-in agent docs to include Qwen Code

Testing

acpx@0.1.3 build:test /Users/user/.openclaw/workspace/acpx
node -e "require('node:fs').rmSync('dist-test',{recursive:true,force:true})" && tsc -p tsconfig.test.json

✔ resolveAgentCommand maps known agents to commands (0.732875ms)
✔ resolveAgentCommand returns raw value for unknown agents (0.063916ms)
✔ listBuiltInAgents returns exactly all 6 registered agent names (0.715416ms)
✔ default agent is codex (0.060125ms)
ℹ tests 4
ℹ suites 0
ℹ pass 4
ℹ fail 0
ℹ cancelled 0
ℹ skipped 0
ℹ todo 0
ℹ duration_ms 83.351042