feat(brainstore): shared BrainStore impl; re-point MCP handlers (#51)

Extracts the #45 write/update/get logic + the wiki upkeep that must
accompany a write (wing _index rebuild, cross-wing auto-tunnel, graph
re-index) into a single concrete brainstore.Store implementing
capture.BrainStore. The MCP brain_write/brain_update/brain_get handlers
are re-pointed at it, so there is one implementation, not two — the DRY
payoff #51 is named for. capture and MCP now share the exact same brain
write path and read-after-write contract.

The Server gains a *brainstore.Store, constructed in NewServer and given
the graph store in WithGraph. Embedding refresh stays out-of-band
(mtime-driven vectorstore.Sync), unchanged. Existing MCP brain_update/
brain_get/brain_write tests pass unmodified — behaviour and the
{id, path, content_hash} response contract are preserved.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ingestion/internal/mcp/server.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"net/http"
 
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brainstore"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/graphstore"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/graphsync"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/pipeline"
@@ -46,6 +47,7 @@ type Server struct {
 	vector    search.VectorSearcher // nil = BM25-only retrieval
 	embedder  search.Embedder       // nil = BM25-only retrieval
 	graph     graphsync.Store       // nil = brain_graph and GraphRAG augmentation disabled
+	store     *brainstore.Store     // shared brain write/update/get impl (also used by capture)
 }
 
 // NewServer constructs a Server bound to brainDir. pipelineCfg supplies the
@@ -56,7 +58,13 @@ func NewServer(brainDir string, pipelineCfg *pipeline.Config, llm pipeline.Compl
 	if pipelineCfg != nil {
 		cfg = *pipelineCfg
 	}
-	return &Server{brainDir: brainDir, pipeline: cfg, llm: llm, answerLLM: answerLLM}
+	return &Server{
+		brainDir:  brainDir,
+		pipeline:  cfg,
+		llm:       llm,
+		answerLLM: answerLLM,
+		store:     brainstore.New(brainDir),
+	}
 }
 
 // WithReranker installs an opt-in cross-encoder reranker. When set,
@@ -84,9 +92,11 @@ func (s *Server) WithHybridRetrieval(v search.VectorSearcher, e search.Embedder)
 func (s *Server) WithGraph(g *graphstore.PGStore) *Server {
 	if g == nil {
 		s.graph = nil
+		s.store.WithGraph(nil)
 		return s
 	}
 	s.graph = g
+	s.store.WithGraph(g)
 	return s
 }