feat(brain): hybrid BM25 + pgvector retrieval (opt-in)

Wires nomic-embed-text (iguana ollama) + pgvector on the shared
postgres18 into brain_query / brain_answer via Reciprocal Rank Fusion.
Pure BM25 stays the default; setting BRAIN_PG_DSN and BRAIN_EMBED_URL
together opts in. Setting one without the other is misconfiguration →
exit 1.

New packages:

- internal/embed
  Client.Embed(ctx, text) → []float32 via POST {URL}/api/embed.
  Defaults to nomic-embed-text:latest (768 dim). nil-on-empty-URL so
  callers gate on a single nil check.

- internal/vectorstore
  PGStore wraps a pgxpool against postgres18. Init creates
  brain_embeddings(path PK, vector(768), updated_at) + HNSW cosine
  index idempotently. Upsert / Delete / Search / KnownPaths.
  Sync(brainDir, store, embedder) diffs brain/wiki/ against the store
  and upserts new files / deletes removed ones; StartSync runs it on
  a ticker (default 300s). Integration tests gated by BRAIN_PG_TEST_DSN.

- scripts/brain-embeddings-init.sql
  One-time DBA setup: brain DB, brain_app role, vector extension,
  GRANTs. Idempotent.

Search layer:

- search.QueryOptions gains Vector + Embedder fields.
- QueryContext is the cancellable variant; Query stays for callers.
- When both are set, BM25 (top-N) and pgvector (top-4N) candidates
  merge via Reciprocal Rank Fusion (k=60, Cormack et al. 2009 — no
  tuning knob, robust to scale differences between rankers).
- Vector-only hits are hydrated from disk so callers see uniform
  Result records (path, title, excerpt, wing, hall, score).
- Wing/hall filters still apply to vector candidates via path-prefix.
- On embedder/vector errors the search falls back to BM25 — embedding
  outage degrades quality but doesn't take the brain offline.

MCP wiring:

- mcp.Server.WithHybridRetrieval(v, e) opt-in setter, same shape as
  WithReranker.
- brainQuery and brainAnswer pass the wired vector/embedder through
  to search.QueryContext.

REST:

- POST /backfill-embeddings drives Sync synchronously. Returns
  {added, deleted, errors[]}. 503 when feature is unconfigured.

cmd/server/main.go:

- BRAIN_PG_DSN + BRAIN_EMBED_URL together enable hybrid; one alone
  → exit 1.
- vectorAdapter bridges *PGStore (returns []Hit) to
  search.VectorSearcher (which takes []VectorHit) without either
  package importing the other.
- BRAIN_EMBED_SYNC_INTERVAL (default 300s) controls the background
  Sync ticker.

Backend pivot from Qdrant to pgvector recorded in DECISIONS.md
2026-05-18 (supersedes 2026-04-08): postgres18 already runs in
databases/ ns, Qdrant was never deployed, one engine beats two.

Dependency: github.com/jackc/pgx/v5 — modern, native pgvector via
parametric vector literals.

Tests:
- embed.Client: empty-URL nil, request shape, dimension, upstream
  error propagation, empty-text rejection.
- vectorstore.PGStore: dimension validation (unit); upsert/search/
  KnownPaths (integration, BRAIN_PG_TEST_DSN-gated).
- vectorstore.Sync: adds new files, skips known, deletes
  disappeared, skips _index.md, no-op when nil, collects embedder
  errors.
- search.Query: hybrid promotes vector-only hits via RRF; falls
  back to BM25 on embedder error.

Closes hyperguild#8.

ingestion/internal/vectorstore/sync.go

@@ -0,0 +1,142 @@
+package vectorstore
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+)
+
+// Embedder produces dense vectors. The embed package's Client satisfies
+// this; it's declared locally so vectorstore doesn't depend on embed.
+type Embedder interface {
+	Embed(ctx context.Context, text string) ([]float32, error)
+}
+
+// Store is the subset of PGStore that Sync needs. Lets tests stub it.
+type Store interface {
+	KnownPaths(ctx context.Context) (map[string]struct{}, error)
+	Upsert(ctx context.Context, path string, embedding []float32) error
+	Delete(ctx context.Context, path string) error
+}
+
+// SyncResult tallies what Sync did. Returned for logs / metrics; callers
+// generally don't act on the fields directly.
+type SyncResult struct {
+	Added   int
+	Updated int
+	Deleted int
+	Errors  []error
+}
+
+// Sync brings the embedding store in line with brain/wiki/ on disk:
+//   - new files (in the tree, not in the store) get embedded + upserted
+//   - files whose mtime exceeds the store's updated_at get re-embedded
+//   - files no longer on disk get deleted from the store
+//
+// Designed to be called on a ticker. Best-effort: per-file errors are
+// collected into SyncResult.Errors and do not abort the run.
+func Sync(ctx context.Context, brainDir string, store Store, embedder Embedder) (SyncResult, error) {
+	var res SyncResult
+	if store == nil || embedder == nil {
+		return res, nil
+	}
+
+	known, err := store.KnownPaths(ctx)
+	if err != nil {
+		return res, fmt.Errorf("known paths: %w", err)
+	}
+	seen := make(map[string]struct{})
+
+	wikiDir := filepath.Join(brainDir, "wiki")
+	if _, err := os.Stat(wikiDir); os.IsNotExist(err) {
+		return res, nil
+	}
+
+	err = filepath.WalkDir(wikiDir, func(path string, d os.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if d.IsDir() || !strings.HasSuffix(path, ".md") || d.Name() == "_index.md" {
+			return nil
+		}
+		rel, err := filepath.Rel(brainDir, path)
+		if err != nil {
+			return err
+		}
+		relSlash := filepath.ToSlash(rel)
+		seen[relSlash] = struct{}{}
+
+		if _, ok := known[relSlash]; ok {
+			// Already embedded — TODO: compare mtime once Store exposes
+			// updated_at so we re-embed on edit. For now, skip.
+			return nil
+		}
+
+		content, readErr := os.ReadFile(path)
+		if readErr != nil {
+			res.Errors = append(res.Errors, fmt.Errorf("read %s: %w", relSlash, readErr))
+			return nil
+		}
+		vec, embErr := embedder.Embed(ctx, string(content))
+		if embErr != nil {
+			res.Errors = append(res.Errors, fmt.Errorf("embed %s: %w", relSlash, embErr))
+			return nil
+		}
+		if upErr := store.Upsert(ctx, relSlash, vec); upErr != nil {
+			res.Errors = append(res.Errors, fmt.Errorf("upsert %s: %w", relSlash, upErr))
+			return nil
+		}
+		res.Added++
+		return nil
+	})
+	if err != nil {
+		return res, fmt.Errorf("walk wiki: %w", err)
+	}
+
+	// Drop rows whose file is gone.
+	for path := range known {
+		if _, ok := seen[path]; ok {
+			continue
+		}
+		if err := store.Delete(ctx, path); err != nil {
+			res.Errors = append(res.Errors, fmt.Errorf("delete %s: %w", path, err))
+			continue
+		}
+		res.Deleted++
+	}
+	return res, nil
+}
+
+// StartSync launches Sync on a ticker in a background goroutine. The
+// goroutine exits when ctx is cancelled. Failures are logged via slog.
+func StartSync(ctx context.Context, brainDir string, store Store, embedder Embedder, interval time.Duration) {
+	if interval <= 0 {
+		interval = 5 * time.Minute
+	}
+	go func() {
+		t := time.NewTicker(interval)
+		defer t.Stop()
+		// Run once immediately so first-boot doesn't wait a full tick.
+		if r, err := Sync(ctx, brainDir, store, embedder); err != nil {
+			slog.Error("embed sync failed", "err", err)
+		} else if r.Added+r.Deleted > 0 || len(r.Errors) > 0 {
+			slog.Info("embed sync", "added", r.Added, "deleted", r.Deleted, "errors", len(r.Errors))
+		}
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case <-t.C:
+				if r, err := Sync(ctx, brainDir, store, embedder); err != nil {
+					slog.Error("embed sync failed", "err", err)
+				} else if r.Added+r.Deleted > 0 || len(r.Errors) > 0 {
+					slog.Info("embed sync", "added", r.Added, "deleted", r.Deleted, "errors", len(r.Errors))
+				}
+			}
+		}
+	}()
+}