Index Forge incidents for listing and stats

[im-sham]

Summary

• Add a compact .forge_incident_index.json for index-backed list, suffix lookup, and aggregate stats.
• Route CLI and MCP incident stats/logging through index-backed helpers.
• Retry generated incident IDs on duplicate saves and lock index updates so concurrent writes do not drop entries.

Audit Findings

• PERF-008: Forge list/show/stats scanned the full incident tree.
• P3-005: date-sequence generated incident IDs could collide under concurrent logging.

Verification

• Red test: focused incident-store test run failed before implementation because get_incident_stats was missing.
• PYTHONPATH=. /Users/shamimrehman/Projects/USMI/forge/.venv/bin/python -m ruff check .
• PYTHONPATH=. /Users/shamimrehman/Projects/USMI/forge/.venv/bin/python -m pytest
• PYTHONPATH=. /Users/shamimrehman/Projects/USMI/forge/.venv/bin/python -m compileall forge_cli tests
• git diff --check