Re-embed stored notes and facts after an embedder swap (#378)

The embedder swap left every stored vector in the old model's space, so cosine against a new query vector is noise. Add the one-shot backfill: store.ReembedAll re-embeds every note and fact text with the currently configured embedder (the passage side, which is the side stored text was written with) and rewrites both places a vector lives — the notes table embedding column and the memory_vectors rows.

All of it plus the embedder marker happens in one transaction, so a failure partway changes nothing and writes no marker: re-run it. A run against a DB whose marker already names the current embedder does nothing.

Triggered explicitly with `mavend -reembed`, not automatically on mismatch: ONNX on the laptop CPU makes this minutes of work, and a silent multi-minute stall on boot would look like a hang. The mismatch warning now tells the user to run it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CGeSZxh1DCtRxmFVSYVGvJ

cmd/mavend/main.go

@@ -189,7 +189,9 @@ func (l *lockedAPI) MorningStatus(ctx context.Context) ([]ipc.MorningRoutineStat
 func run(args []string) error {
 	cfgPath := flag.String("config", defaultConfigPath(), "path to mavend JSON config")
 	wrappedKeyPath := flag.String("wrapped-key-file", "", "path to wrapped encryption key blob (enables cold-start unlock)")
+	reembed := flag.Bool("reembed", false, "re-embed every stored note and fact with the configured embedder, then serve normally (run once after an embedder swap)")
 	flag.CommandLine.Parse(args)
+	reembedOnStart = *reembed
 	cfg, err := config.Load(*cfgPath)
 	if err != nil {
 		return err