feat(tracing): auto-detect git information

[Naoray]

Summary

• Auto-detect git information (hash, branch, tag, dirty status) in the EnvironmentCollector using proc_open with a 1-second timeout
• New GitInfoDetector class with static caching so git commands run only once per process
• Graceful fallback when git is unavailable or not in a git repo (fields are simply omitted)
• config('app.git_commit') still works as an override for git_hash
• New git toggle under tracing config (default: true)
• Backward compatible: git_commit key is preserved in environment data

Closes #38

Test plan

• Existing EnvironmentCollector test still passes
• Git data is included in environment context when available (mocked)
• Graceful fallback when git is not available returns empty git info
• config('app.git_commit') override takes precedence over auto-detected hash
• Git detection is skipped when tracing.git config is false
• git_commit backward-compatible key is set from git_hash
• Git tracing is enabled by default
• GitInfoDetector integration tests verify real git commands work
• Static cache works correctly across multiple calls
• Full test suite passes (289 tests, 914 assertions)

🤖 Generated with Claude Code

[Naoray]

Do we really need this? Does it add value?

[Naoray]

I think this does add value - knowing the deployed commit hash and branch when an error occurs is useful for debugging. I've simplified the implementation by replacing the raw proc_open with Laravel's Process facade, reducing runGitCommand() from ~50 lines to 3. The static cache ensures git commands only run once per process.