Smoke-test AgentTX with OpenCode and document the result

[aliengineering-byte]

Run a real, harmless AgentTX transaction with a current OpenCode CLI and document the result without overstating support.

Suggested workflow:

1. Create a disposable Git repository with one committed text file.
2. Record OpenCode, OS, Node, Git, and AgentTX versions.
3. Run agenttx run opencode and ask it to make one harmless tracked-file edit.
4. Confirm interactive input and child exit behave normally.
5. Run agenttx diff, agenttx inspect, and agenttx rollback.
6. Confirm the original repository stayed unchanged and clean.
7. Add the dated evidence to docs/AGENT_COMPATIBILITY.md, including limitations or failures.

Acceptance criteria:

• no credentials, prompts, private source, or private paths in fixtures or logs;
• exact version, platform, and date recorded;
• both successful and failed behavior reported honestly;
• no claim of broad OpenCode compatibility from a single smoke test.