feat: shadow AI detection for repositories

[Siddhant-K-code]

Closes #46

What

Adds agent-strace audit-tools — scans a git repository for AI tool usage signatures using file presence and commit message patterns. No network calls required; git-only by default.

Usage

$ agent-strace audit-tools --repo . --approved "Claude Code"

AI Tool Usage Report
───────────────────────────────────────────────────────
Repo:  /path/to/repo
Since: 90 days ago
Approved: Claude Code
───────────────────────────────────────────────────────
Detected tools:

  ✅ Claude Code          (confirmed)
       file present: CLAUDE.md
  ⚠️  Cursor              (confirmed)
       file present: .cursorrules

Unapproved tool signals:
  ❌ Cursor detected but not in approved list
───────────────────────────────────────────────────────

Detection signals (no LLM calls, pure static analysis)

Tool	File signals	Commit patterns
Claude Code	CLAUDE.md, .claude/settings.json	\bClaude\b in message
Cursor	.cursorrules, .cursor/	cursor ai in message
GitHub Copilot	.github/copilot-instructions.md	Co-authored-by: GitHub Copilot
Codex / ChatGPT	codex.md, .openai/	generated by chatgpt
Windsurf	.windsurfrules, .codeium/	—
Aider	.aider.conf.yml, .aider/	—

Confidence levels: confirmed (file present) > likely (commit patterns only).

Changes

src/agent_trace/shadow_ai.py (new)

• detect_ai_tools(): main detection function
• format_audit_tools(): report formatter
• _file_exists_in_repo(): checks file/dir presence
• _scan_commit_messages(): regex scan of git log
• _scan_env_history(): scans .env git history for LLM endpoints
• _scan_pii_in_commits(): detects PII patterns in recent commits
• cmd_audit_tools(): CLI handler

src/agent_trace/cli.py

• audit-tools subcommand with --repo, --since, --approved

tests/test_shadow_ai.py: 9 tests covering file detection, approval filtering, confidence levels, and output formatting.