fix: codex stop hook guidance output

[obrunogonzaga]

Summary

• Map Codex Stop workflow guidance to systemMessage instead of hookSpecificOutput.
• Keep Codex SessionStart guidance behavior unchanged.
• Update dispatch and round-trip tests for the Codex Stop contract.

Root cause

Codex rejects active Stop hook stdout that only contains hookSpecificOutput. With active PREVC guidance, dotcontext emitted that shape, so Codex reported hook returned invalid stop hook JSON output.

Validation

• npx jest src/integrations/__tests__/hookRoundTrip.test.ts --runInBand
• npx jest src/cli/services/__tests__/hookDispatchService.test.ts --runInBand
• npm test -- --runInBand
• npm run build

Fixes #74

[vinilana]

We should prevent this to happen to other harnesses as well, this is why it was a generic message, we should map to Codex output and so others instead of only covering it here for Codex, because we support other tools, @cursoragent can you help us here?