enrich: backfill filename dedup key on content-hash hit

Transcripts mined before the filename-key feature was added had
content-hash keys (#h-) but no filename keys (#f-). The daemon's
fast-path check only looks at filename keys, so these sessions were
re-queued every tick, hitting the content-hash dedup (0.0s) but
returning early before writing the filename key — a self-perpetuating
loop burning Sonnet quota on ~560 phantom re-mines per minute.

Fix: when the content-hash dedup fires and no filename key exists,
backfill it before returning.

src/enrich.rs

@@ -274,6 +274,15 @@ pub fn experience_mine(
     let dedup_key = format!("_mined-transcripts.md#h-{:016x}", hash);
 
     if store.nodes.contains_key(&dedup_key) {
+        // Backfill filename key if missing (transcripts mined before this key existed)
+        let fname_key = transcript_filename_key(jsonl_path);
+        if !store.nodes.contains_key(&fname_key) {
+            let mut node = new_node(&fname_key, &format!("Backfilled from {}", dedup_key));
+            node.category = store::Category::Task;
+            node.provenance = store::Provenance::AgentExperienceMine;
+            let _ = store.upsert_node(node);
+            store.save()?;
+        }
         println!("  Already mined this transcript ({}), skipping.", &dedup_key[24..]);
         return Ok(0);
     }