[codex] Make repository scans gitignore-aware

[rickardvh]

Summary

• add a central repository_scan_files helper that uses git ls-files with --exclude-standard when available
• migrate reuse-pressure, repo-friction, and artifact-footprint scans to the shared helper
• keep untracked non-ignored files visible, keep managed .agentic-workspace files visible, and skip .agentic-workspace/local/scratch for reuse-pressure

Closes #1214.

Validation

• uv run pytest tests/test_repository_scanning.py tests/test_workspace_implement_cli.py tests/test_workspace_report_cli.py tests/test_workspace_summary_cli.py::test_external_intent_refresh_applies_stale_candidate_reconciliation_and_preserves_delta -q
• uv run agentic-workspace summary --target . --format json
• uv run agentic-workspace doctor --target . --modules planning --format json
• make test-workspace
• make lint-workspace

Stack

• stacked on [codex] Isolate generated package proof tests #1219 / codex/1212-windows-static-proof-stability so the Windows static-proof stability fix is present while validating this branch.