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_test.go

@@ -0,0 +1,85 @@
+package brainstore_test
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brainstore"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/capture"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestStoreWriteReturnsHandle(t *testing.T) {
+	dir := t.TempDir()
+	s := brainstore.New(dir)
+
+	ref, err := s.Write(context.Background(), capture.Note{
+		Content: "# X\n\nbody\n", Filename: "x", Wing: "a", Hall: "facts",
+	})
+	require.NoError(t, err)
+	assert.Equal(t, "wiki/a/facts/x.md", ref.Path)
+	assert.Equal(t, ref.Path, ref.ID)
+	assert.NotEmpty(t, ref.ContentHash)
+	assert.False(t, ref.Superseded)
+
+	_, err = os.Stat(filepath.Join(dir, "wiki/a/facts/x.md"))
+	require.NoError(t, err)
+}
+
+func TestStoreUpdateSupersedes(t *testing.T) {
+	dir := t.TempDir()
+	s := brainstore.New(dir)
+	_, err := s.Write(context.Background(), capture.Note{
+		Content: "old\n", Filename: "n", Wing: "a", Hall: "facts",
+	})
+	require.NoError(t, err)
+
+	ref, err := s.Update(context.Background(), "n", capture.Note{
+		Content: "new\n", Wing: "a", Hall: "facts", Reason: "changed",
+	})
+	require.NoError(t, err)
+	assert.True(t, ref.Superseded)
+	assert.Equal(t, "wiki/a/facts/n.md", ref.Path)
+
+	got, _ := os.ReadFile(filepath.Join(dir, "wiki/a/facts/n.md"))
+	assert.Contains(t, string(got), "new")
+	assert.Contains(t, string(got), "supersede_reason: changed")
+}
+
+func TestStoreUpdateByFullPath(t *testing.T) {
+	dir := t.TempDir()
+	s := brainstore.New(dir)
+	_, err := s.Write(context.Background(), capture.Note{Content: "old\n", Filename: "n", Wing: "a", Hall: "facts"})
+	require.NoError(t, err)
+
+	ref, err := s.Update(context.Background(), "wiki/a/facts/n.md", capture.Note{Content: "fresh\n"})
+	require.NoError(t, err)
+	assert.Equal(t, "wiki/a/facts/n.md", ref.Path)
+}
+
+func TestStoreUpdateMissingErrors(t *testing.T) {
+	dir := t.TempDir()
+	s := brainstore.New(dir)
+	_, err := s.Update(context.Background(), "ghost", capture.Note{Content: "x\n", Wing: "a", Hall: "facts"})
+	require.Error(t, err)
+	_, statErr := os.Stat(filepath.Join(dir, "wiki/a/facts/ghost.md"))
+	assert.True(t, os.IsNotExist(statErr), "update must not create")
+}
+
+func TestStoreGetRoundTripsHash(t *testing.T) {
+	dir := t.TempDir()
+	s := brainstore.New(dir)
+	ref, err := s.Write(context.Background(), capture.Note{
+		Content: "# Body\n\ntext\n", Filename: "n", Wing: "a", Hall: "facts",
+	})
+	require.NoError(t, err)
+
+	note, err := s.Get(context.Background(), ref.ID)
+	require.NoError(t, err)
+	assert.Equal(t, ref.ContentHash, note.ContentHash, "write→get hash round-trips")
+	assert.Equal(t, "a", note.Frontmatter["wing"])
+	assert.Contains(t, note.Body, "# Body")
+}