feat(agent): /clear now clears seahorse DB in addition to JSONL

- Add Clear(ctx, sessionKey) to ContextManager interface
- Implement Clear for legacy (JSONL) and seahorse (DB + JSONL)
- Add Engine.ClearSession + Store.ClearConversation
- Fix FTS5 DELETE trigger syntax in schema (was using wrong
  external-content FTS5 syntax; now uses standard DELETE FROM)
- Fix ClearSession to skip sessions never ingested (was creating
  blank conversations record via GetOrCreateConversation)
- Simplify summary_parents DELETE into single OR statement
- Add TestStoreClearConversation unit test

pkg/agent/context_manager.go

@@ -24,6 +24,10 @@ type ContextManager interface {
 	// Ingest records a message into the ContextManager's own storage.
 	// Called after each message is persisted to session JSONL.
 	Ingest(ctx context.Context, req *IngestRequest) error
+
+	// Clear removes all stored context for a session (messages, summaries, etc.).
+	// Called when the user issues /clear or /reset.
+	Clear(ctx context.Context, sessionKey string) error
 }
 
 // AssembleRequest is the input to Assemble.