Sessions: semantic session_search over shared embedding backend

Phase 2: route the session VectorIndex through internal/embedding so
session_search gains real semantic matching when an HTTP embeddings
backend (Ollama/OpenAI/etc.) is configured, instead of being hardwired to
go-vector RandomProjections.

  - VectorIndex.emb is now embedding.TextEmbedder, selected by an
    embedding.Config. Default (nil) stays RandomProjections — unchanged
    for existing users.
  - Add a vectors_meta.json fingerprint file next to vectors.gob. On init
    the persisted vectors are only reused when the embedder fingerprint
    (provider/model/dims) matches; switching backends forces a one-time
    rebuild in the new space. A missing meta (legacy layout) also rebuilds,
    since the old raw-text RP vectors live in a different space than the
    shared featurized embedder.
  - Rebuild batch-embeds the whole corpus via EmbedAll (one HTTP call) and
    adopts the episode index's 30s failure cool-down so a down backend is
    not hammered on every search or session save.
  - Resilience: Add never fails a session save and Search never surfaces an
    error when the backend is unavailable — both degrade silently to the
    keyword fallback in session_search_tool.go (left untouched).

Wiring: Store.InitVectorIndex takes an *embedding.Config; the continue and
telegram entry points pass memory's embedding config so a single endpoint
powers both subsystems (a dedicated sessions block lands in Phase 4).

Tests mirror memory's: HTTP semantic match (kitten→feline), fingerprint
invalidation forcing a rebuild, and the rebuild backoff under a down backend.

Author: claude

cmd/odek/main.go

@@ -1640,8 +1640,6 @@ func continueCmd(args []string) error {
 	if err != nil {
 		return fmt.Errorf("session store: %w", err)
 	}
-	// Initialize semantic search index (non-fatal on failure).
-	_ = store.InitVectorIndex()
 
 	var sess *session.Session
 	if sessionID != "" {
@@ -1659,6 +1657,10 @@ func continueCmd(args []string) error {
 	// Resolve config (no CLI flags for continue — uses session's model)
 	resolved := config.LoadConfig(config.CLIFlags{Model: sess.Model})
 
+	// Initialize semantic search index (non-fatal on failure). Sessions share
+	// memory's embedding backend so one endpoint config powers both.
+	_ = store.InitVectorIndex(resolved.Memory.Embedding)
+
 	// Auto-apply sandbox if session was sandboxed (even if config changed)
 	if sess.Sandbox && !resolved.Sandbox {
 		resolved.Sandbox = true

cmd/odek/telegram.go

@@ -143,8 +143,9 @@ func telegramCmd(args []string) error {
 		return err
 	}
 
-	// Initialize semantic search index.
-	if err := store.InitVectorIndex(); err != nil {
+	// Initialize semantic search index. Sessions share memory's embedding
+	// backend so a single Ollama/OpenAI endpoint config powers both.
+	if err := store.InitVectorIndex(resolved.Memory.Embedding); err != nil {
 		fmt.Fprintf(os.Stderr, "odek telegram: vector index: %v\n", err)
 		// Non-fatal — search falls back to metadata-only.
 	}

internal/session/session.go

@@ -26,6 +26,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/BackendStack21/odek/internal/embedding"
 	"github.com/BackendStack21/odek/internal/llm"
 	"github.com/BackendStack21/odek/internal/redact"
 )
@@ -74,15 +75,16 @@ func NewStore() (*Store, error) {
 	return &Store{dir: dir}, nil
 }
 
-// InitVectorIndex initializes the semantic search index. Must be called
+// InitVectorIndex initializes the semantic search index using the embedding
+// backend selected by cfg (nil = default RandomProjections). Must be called
 // after NewStore, before the first Save. Safe to call multiple times —
-// subsequent calls are no-ops.
-func (s *Store) InitVectorIndex() error {
+// subsequent calls are no-ops once the index is ready.
+func (s *Store) InitVectorIndex(cfg *embedding.Config) error {
 	if s.Vec != nil && s.Vec.Ready() {
 		return nil // already initialized
 	}
 	s.Vec = new(VectorIndex)
-	return s.Vec.Init(s.dir)
+	return s.Vec.InitWithConfig(s.dir, cfg)
 }
 
 // ── ID Generation ──────────────────────────────────────────────────────