fix(nodb): set storeActive when initializing no-db mode

The `bd comment --no-db` command was failing because
initializeNoDbMode() set the global `store` but never set
`storeActive = true`. When comments.go called ensureStoreActive(),
the guard check failed and it tried to find a SQLite database,
returning an ironic error telling the user to use --no-db.

Why only `comment` was affected:
- Commands like `create`, `update`, `close` use `store` directly
- The `comment` command calls `ensureStoreActive()` first as a safety check
- That function guards on `storeActive && store != nil`
- Since `storeActive` was never set, the guard failed and it looked for SQLite

The fix aligns no-db mode with what ensureStoreActive() expects.

cmd/bd/nodb.go

@@ -72,8 +72,11 @@ func initializeNoDbMode() error {
 
 	debug.Logf("using prefix '%s'", prefix)
 
-	// Set global store
+	// Set global store and mark as active (fixes bd comment --no-db)
+	storeMutex.Lock()
 	store = memStore
+	storeActive = true
+	storeMutex.Unlock()
 	return nil
 }