feat: add execution timings and job summaries

[machado144]

Summary

• add compact per-gate timings to agent and GitHub verdict lines
• append escaped gate status, duration, and finding counts to GITHUB_STEP_SUMMARY
• keep summary I/O failures nonfatal and routed through CLI stderr
• add Spec 009, edge coverage, and summary-environment isolation

Validation

• go test -race ./...
• make test-integration with writable Go and golangci caches
• golangci-lint run --timeout=5m
• built bin/gauntlet produced one timed GitHub summary table

Spec: docs/specs/009-execution-timings.md
Plan: docs/specs/009-execution-timings.plan.md

[github-actions]

✅ PR title follows the required format

Current title: feat: add execution timings and job summaries