feat: emit agent usage metrics even when not in a git repo

[svarlamov]

feat: emit agent usage metrics even when not in a git repo

Summary

When git-ai checkpoint <agent> is invoked outside a git repository, the process previously exited early without recording any metrics. This PR ensures the AgentUsage metric event is still emitted at the two no-repo exit paths in handle_checkpoint:

1. Multi-repo file detection finds no repositories for the edited files
2. No edited files provided and working directory is not a git repo

A new emit_no_repo_agent_metrics helper in git_ai_handlers.rs builds EventAttributes with the available agent info (tool, model, prompt_id) but without repo-specific fields (repo_url, branch, base_commit_sha), records the AgentUsage event, and triggers a background flush.

should_emit_agent_usage in checkpoint.rs was changed from private to pub(crate) so the rate-limiting logic can be reused.

Review & Testing Checklist for Human

• Verify spawn_background_flush vs process::exit(0) timing: The metric is written synchronously to the local log file via record(), but spawn_background_flush() is called right before std::process::exit(0). Confirm the background flush either completes or that the local log file will be picked up on the next flush cycle — otherwise the metric may never reach the server.
• Confirm only AgentUsage is needed (not per-file Checkpoint events): This PR does not emit Checkpoint (event ID 4) events in the no-repo path since per-file line stats can't be computed without a repository. Verify this matches the intended requirement.
• Nullable attrs on server side: The AgentUsage events emitted here will lack repo_url, branch, and base_commit_sha. Confirm the metrics ingestion server handles these as nullable without errors.
• Test manually: Run git-ai checkpoint claude --hook-input '<valid_json>' from a non-git directory and verify an AgentUsage event appears in the local observability log.

Notes

• Link to Devin run
• Requested by: @svarlamov

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.