store: add reindex_relations for after Vec mutations

- Add index::clear_relations() to drop and recreate RELS table
- Add Store::reindex_relations() to rebuild index from Vec
- Call reindex_relations() at end of dedup command

This ensures index stays in sync with Vec after complex mutations
like UUID redirection in dedup. Vec mutations remain for now but
index is correctly updated afterward.

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/hippocampus/store/mod.rs

@@ -130,6 +130,18 @@ impl Store {
         Ok(())
     }
 
+    /// Rebuild relation index from Vec. Call after mutations that modify relations.
+    pub fn reindex_relations(&self) -> Result<()> {
+        if let Some(db) = self.db.as_ref() {
+            index::clear_relations(db)?;
+            for rel in &self.relations {
+                if rel.deleted { continue; }
+                index::index_relation(db, &rel.source, &rel.target, rel.strength, rel.rel_type as u8)?;
+            }
+        }
+        Ok(())
+    }
+
     pub fn resolve_key(&self, target: &str) -> Result<String> {
         // Strip .md suffix if present — keys no longer use it
         let bare = strip_md_suffix(target);