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/brainstore/store.go

@@ -0,0 +1,129 @@
+// Package brainstore is the concrete BrainStore: the single shared
+// implementation of the #45 write/update/get verbs, used by BOTH the MCP
+// handlers and the capture use-case so there is one implementation, not
+// two (the Clean-Architecture / DRY payoff of #51).
+//
+// It composes the file-level primitives in package api (WriteNote,
+// UpdateNote, ReadNote — the read-after-write contract) with the wiki
+// upkeep that must accompany a write: wing _index rebuild, cross-wing
+// auto-tunnel, and graph re-index. Embedding refresh is intentionally
+// out-of-band (mtime-driven vectorstore.Sync) and not triggered here —
+// see the brain note on out-of-band sync.
+package brainstore
+
+import (
+	"context"
+	"log/slog"
+	"strings"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/api"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brain"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/capture"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/graphsync"
+)
+
+// Store implements capture.BrainStore against a brain directory on disk,
+// optionally re-indexing each write into the knowledge graph.
+type Store struct {
+	brainDir string
+	graph    graphsync.Store // nil = graph re-index disabled
+}
+
+// New constructs a Store bound to brainDir with graph indexing disabled.
+func New(brainDir string) *Store {
+	return &Store{brainDir: brainDir}
+}
+
+// WithGraph enables graph re-index on every write/update. nil disables it.
+func (s *Store) WithGraph(g graphsync.Store) *Store {
+	s.graph = g
+	return s
+}
+
+// Write creates a brain note and returns its read-after-write handle.
+func (s *Store) Write(ctx context.Context, n capture.Note) (capture.Ref, error) {
+	relPath, err := api.WriteNote(s.brainDir, api.WriteNoteOptions{
+		Content:  n.Content,
+		Filename: n.Filename,
+		Type:     n.Type,
+		Domain:   n.Domain,
+		Wing:     n.Wing,
+		Hall:     n.Hall,
+	})
+	if err != nil {
+		return capture.Ref{}, err
+	}
+	s.wikiUpkeep(relPath, n.Wing, n.Content)
+	s.indexInGraph(ctx, "brain_write", relPath)
+
+	_, _, hash, _ := api.ReadNote(s.brainDir, relPath)
+	return capture.Ref{ID: relPath, Path: relPath, ContentHash: hash}, nil
+}
+
+// Update supersedes an existing note in place. slug may be a bare slug
+// (resolved against n.Wing/n.Hall) or a full brain-relative path (when it
+// contains a slash). It never creates — a missing target is an error.
+func (s *Store) Update(ctx context.Context, slug string, n capture.Note) (capture.Ref, error) {
+	opts := api.UpdateNoteOptions{Content: n.Content, Reason: n.Reason}
+	if strings.Contains(slug, "/") {
+		opts.Path = slug
+	} else {
+		opts.Wing, opts.Hall, opts.Slug = n.Wing, n.Hall, slug
+	}
+
+	relPath, hash, _, err := api.UpdateNote(s.brainDir, opts)
+	if err != nil {
+		return capture.Ref{}, err
+	}
+	if wing := wingFromRelPath(relPath); wing != "" {
+		s.wikiUpkeep(relPath, wing, n.Content)
+	}
+	s.indexInGraph(ctx, "brain_update", relPath)
+
+	return capture.Ref{ID: relPath, Path: relPath, ContentHash: hash, Superseded: true}, nil
+}
+
+// Get fetches a note by id/path — the read-after-write confirmation
+// primitive (a direct fetch, never a semantic query).
+func (s *Store) Get(_ context.Context, id string) (capture.StoredNote, error) {
+	fm, body, hash, err := api.ReadNote(s.brainDir, id)
+	if err != nil {
+		return capture.StoredNote{}, err
+	}
+	return capture.StoredNote{ID: id, Path: id, ContentHash: hash, Frontmatter: fm, Body: body}, nil
+}
+
+// wikiUpkeep rebuilds the wing _index and re-tunnels cross-wing matches
+// when a note lands in the structured wiki. Both are best-effort: the
+// note is already written, so a failure here is logged, not propagated.
+func (s *Store) wikiUpkeep(relPath, wing, content string) {
+	if wing == "" {
+		return
+	}
+	if err := brain.BuildWingIndex(s.brainDir, wing); err != nil {
+		slog.Warn("brainstore: auto-index failed", "wing", wing, "err", err)
+	}
+	if err := brain.AutoTunnel(s.brainDir, relPath, content); err != nil {
+		slog.Warn("brainstore: auto-tunnel failed", "src", relPath, "err", err)
+	}
+}
+
+// indexInGraph re-indexes a written doc into the graph, best-effort.
+func (s *Store) indexInGraph(ctx context.Context, op, relPath string) {
+	if s.graph == nil || relPath == "" {
+		return
+	}
+	if err := graphsync.IndexDoc(ctx, s.graph, s.brainDir, relPath); err != nil {
+		slog.Warn(op+": graph index failed", "path", relPath, "err", err)
+	}
+}
+
+// wingFromRelPath extracts the wing from a structured wiki path
+// (wiki/<wing>/<hall>/<slug>.md). Returns "" for legacy/non-wiki paths.
+func wingFromRelPath(relPath string) string {
+	parts := strings.Split(relPath, "/")
+	if len(parts) >= 4 && parts[0] == "wiki" {
+		return parts[1]
+	}
+	return ""
+}