v0.6.0: replace retired gemini agent with agy (Antigravity CLI) + integration tests

[Codevena]

Summary

Google retires the Gemini CLI on 2026-06-18; its successor is the Antigravity CLI (agy). This PR swaps the agent, hardens the pipeline, and adds a deterministic offline integration test suite — releasing as v0.6.0.

• agy agent replaces gemini (breaking: gemini now exits with a migration message). Invocation details verified against the real CLI: --add-dir <project> (agents launch from the operator's CWD), --print-timeout pinned 60s above the fixbuddy watchdog (agy exits 0 on its internal timeout — also detected via output and reclassified as rc=124 → fix:blocked/auto-requeue), --sandbox on verify/review as defense in depth (agy has no read-only mode).
• Integration tests (tests/integration.sh, 14 scenarios): stub gh/agent CLIs + a local bare repo as origin, so branches/commits/pushes are real. Covers happy path, false positive, review rejection, check gate, dry-run read-only, crash classification, the agy invocation contract, and the new read-only-stage guards. Wired into CI.
• Read-only-stage guards (found by the review gate, fixed via TDD): verify-stage worktree residue is stashed and base-branch commits discarded on every verify outcome; the review branch is pinned to the reviewed commit; reviewer worktree residue is stashed on every review outcome (targeted stash pop by message).
• Docs/release: wizard, README, action.yml, CONTRIBUTING, CHANGELOG, version bumps, fresh SHA256SUMS, findings.md → docs/audit/.

Test Plan

• bash -n (each script individually) + shellcheck (default severity, mirrors CI) clean
• tests/integration.sh: 14 passed, 0 failed (offline, no API keys)
• Review gate: Codex PASS + Claude PASS (zero CRITICAL/WARN)
• CI green on this PR (shell checks, integration tests, action-smoke dry-run)

Release steps after merge

1. git tag v0.6.0 && git push origin v0.6.0
2. git tag -f v1 v0.6.0 && git push origin v1 --force

🤖 Generated with Claude Code