D36 — mcpserver: unified Deps registration, split per domain, shared index

Decision. Three structural interventions on internal/mcpserver (July 2026): 1. Split per domain: tools.go (2548 lines) is split into tools_read.go, tools_search.go, tools_write.go, tools_governance.go, tools_skill.go, tools_sync.go; only registration and shared helpers remain in tools.go. 2. Unified registration: the 5 RegisterKBTools* variants are replaced by a single RegisterKBTools(s, k, deps Deps) with Deps{Embedder, VecStore, SQLIndex, BundleFS} (nil fields = capability absent). search/index_rebuild are also a single implementation driven by deps (the response mode fields remain keyword/hybrid/keyword_fts5/hybrid_fts5). 3. Shared keyword index: the double pointer **search.Index is replaced by the concurrency-safe liveIndex wrapper (RWMutex — over HTTP, tools may run concurrently), shared between concept_write and search in all paths. concept_write also updates the persisted FTS5 index (SQLIndex.Upsert best-effort). This supersedes the exception documented in D24: writes are immediately searchable in the embed/SQLite paths too.

Additionally, main.go now passes SQLIndex always (no longer only with Ollama active): FTS5 substring keyword search is available even without an embedder, as intended by D32. use_semantic=true with no embedder configured returns an explicit application error instead of a latent panic.

Rationale. The 5 registration functions grew combinatorially with each optional capability; the Deps struct makes adding a capability O(1). The triplication of search/index_rebuild duplicated logic that silently diverged (the D24 exception was the symptom). The tools.go monolith made navigation expensive; the split follows the same categories as control-plane.md §MCP API.