docs: consolidate documentation (15 → 11 files)

Remove stale and thin docs:
- PERSISTENT_MEMORY_SYSTEM_SPEC.md (875 lines, self-declared legacy)
- docs/2026-04-06-memoria-llm-wiki-integration-plan.md (912 lines, executed plan)
- docs/SKILL_USAGE.md (58 lines, just a pointer to SKILL.md)
- docs/CONTAINER.md (35 lines, merged into docs/INSTALL.md)

Also update docs/INSTALL.md version references to v1.9.0.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: raybird