[codex] Add OpenCode harness support

[firewalker06]

Summary

• Add OpenCode as a built-in managed-agent harness, including setup/readiness detection, CLI and Remote UI exposure, command construction, session resume, and skill discovery paths.
• Add an OpenCode JSONL parser plus structured result extraction support for text, tool use, token/cost, permission-denied, resume, and fenced JSON result events.
• Fix stale PR diff cache detection by switching cached PR diff snapshots to github_diff_v2 and fetching current diffs through gh pr diff.

Validation

• git diff --check
• bin/test
• OpenCode CLI E2E smoke with temp Tycho config/log roots, verifying create/run/send, native sessionID reuse, memory persistence, and structured result output.