feat(memory): optional SQLite connection open timeout

- Add memory.sqlite_open_timeout_secs config (None = wait indefinitely).
- When set, open the DB in a thread with recv_timeout; cap at 300s.
- Default remains None for backward compatibility.
- Document in README; add tests for timeout path and default.

src/config/schema.rs

@@ -781,6 +781,12 @@ pub struct MemoryConfig {
     /// Auto-hydrate from MEMORY_SNAPSHOT.md when brain.db is missing
     #[serde(default = "default_true")]
     pub auto_hydrate: bool,
+
+    // ── SQLite backend options ─────────────────────────────────
+    /// For sqlite backend: max seconds to wait when opening the DB (e.g. file locked).
+    /// None = wait indefinitely (default). Recommended max: 300.
+    #[serde(default)]
+    pub sqlite_open_timeout_secs: Option<u64>,
 }
 
 fn default_embedding_provider() -> String {
@@ -845,6 +851,7 @@ impl Default for MemoryConfig {
             snapshot_enabled: false,
             snapshot_on_hygiene: false,
             auto_hydrate: true,
+            sqlite_open_timeout_secs: None,
         }
     }
 }
@@ -2279,6 +2286,7 @@ default_temperature = 0.7
         assert_eq!(m.archive_after_days, 7);
         assert_eq!(m.purge_after_days, 30);
         assert_eq!(m.conversation_retention_days, 30);
+        assert!(m.sqlite_open_timeout_secs.is_none());
     }
 
     #[test]