Add GitHub Actions CI on agent/* branches with issue comment on result

[infraax]

Task: Agent Branch CI

Parent epic: #13 (Infrastructure)

What to do

Create .github/workflows/agent-ci.yml — runs on every push to agent/* branches.

What it does

1. Runs make test (Go)
2. Runs cd python && pytest (Python)
3. Reads Agent-Name, Agent-Model, Session-Id, Issue trailers from the triggering commit
4. Posts a pass/fail comment on the referenced GitHub issue

Comment format (on success)

✅ CI passed on `agent/claude-code-vscode/issue-42`

| Check | Result |
|---|---|
| Go tests | ✓ passing |
| Python tests | ✓ 24 passing |
| Commit | `abc1234` |
| Agent | `claude-code-vscode` / `claude-sonnet-4-6` |
| Session | `20260325-143022-a3f9` |

Comment format (on failure)

❌ CI failed on `agent/claude-code-vscode/issue-42`
[failure details]

Implementation notes

• Use actions/checkout@v4, actions/setup-go@v5, actions/setup-python@v5
• Extract trailers with: git log -1 --format='%(trailers:key=Issue,valueonly)'
• Post comment using actions/github-script or gh issue comment
• Only run on agent/** branch pattern — not main
• Cache Go module cache (use .gomodcache path) and Python venv

Definition of done

Push to an agent/* branch → CI runs → comment appears on the linked issue within 3 minutes.