docs: add AGENTS_INDEX.md (Layer 1 agent operations index)
Adds AGENTS_INDEX.md — agent-friendly navigation index following lessie-org-map schema v2.
Highlights:
- 100% repo-root resolvable
file:linecitations - Debug Entrypoint Candidates (≥3, covering perf/correctness/availability)
- Stress-query tested
- Coexists with existing
CLAUDE.md/README.md(does not replace them)
Generated by Claude Code (opus) using lessie-org-map/scripts/layer1-prompt.md v2.1 + fix-paths.py.
Part of Lessie multi-repo Layer 1 documentation rollout (13 repos).