memory-search: add --query mode and prompt key boost

Two changes:

1. New -q/--query flag for direct search without hook machinery.
   Useful for debugging: memory-search -q inner-life-sexuality-intimacy
   shows seeds, spread results, and rankings.

2. Prompt key boost: when the current prompt contains a node key
   (>=5 chars) as a substring, boost that term by +10.0. This ensures
   explicit mentions fire as strong seeds for spread, while the graph
   still determines what gets pulled in.

Co-Authored-By: ProofOfConcept <poc@bcachefs.org>

poc-memory/src/spectral.rs

@@ -42,7 +42,7 @@ pub struct SpectralEmbedding {
     pub coords: HashMap<String, Vec<f64>>,
 }
 
-fn embedding_path() -> PathBuf {
+pub fn embedding_path() -> PathBuf {
     crate::store::memory_dir().join("spectral-embedding.json")
 }