docs: reorganize docs landscape
Implements the 4-phase documentation reorganization master plan.
- Consolidation: Merged brief/, handoff/, planning/, and decisions/ into briefs/, handoffs/, plans/, and adr/ respectively (101 ADRs relocated)
- Root Cleanup: Relocated HANDOFF-gpt55-*.md and key top-level docs (runtime_contracts.md, etc.) to canonical folders. Added superseded alerts.
- Indices & Navigation: Created docs/README.md navigation document, docs/sessions/README.md index, docs/adr/README.md index
- Note: Also includes prior commit adding ADR-0200+ corpus hygiene governance (ADR-0225, dependency map, backfilled cross-references)
54e6bfc0d0