feat(brain): add wiki compiler, synthesis, and brain API routes

## What & Why
Wire Phase 5–6 compiled wiki and query synthesis into docs brains: post-ingest
wiki compile, graph expansion signal, knowledge_synthesize tool, and REST endpoints.

## Requirements
- FR-1: Two-step WikiCompiler writes compiled pages under brain storage wiki/.
- FR-2: Wiki graph provides 4-signal expansion for hybrid_graph retrieval mode.
- FR-3: synthesize_brain_query returns cited answers with gap analysis via LLM.
- FR-4: Expose wiki pages, purpose.md, compile, synthesize, and maintenance APIs.
- FR-5: Register knowledge_synthesize tool gated by brain synthesis.enabled config.
- FR-6: Auto-trigger wiki compile after successful ingest when wiki_compiler enabled.
- FR-7: Purge compiled wiki pages when source material is deleted.
- NFR-1: hybrid_graph falls back gracefully when wiki directory is empty.
- AC-1: knowledge_synthesize returns error when synthesis is disabled on mounted brains.
- AC-2: meta_agent prompt mentions knowledge_synthesize when synthesis is enabled.

Plan-Id: 2026-06-02-near-local-kb-build-and-retrieval-quality
Plan-File: .cursor/plans/2026-06-02-near-local-kb-build-and-retrieval-quality.plan.md
Made-with: Damon Li