session: --session flag, journal agent cycle, conversation bytes config

- memory-search: add --session flag for multi-session support
- config: add surface_conversation_bytes option
- journal_agent_cycle: trigger journal agent every 10KB of conversation
- Session::from_id() constructor

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

src/config.rs

@@ -88,6 +88,9 @@ pub struct Config {
     /// Surface agent timeout in seconds.
     #[serde(default)]
     pub surface_timeout_secs: Option<u32>,
+    /// Max conversation bytes to include in surface agent context.
+    #[serde(default)]
+    pub surface_conversation_bytes: Option<usize>,
     /// Hook events that trigger the surface agent.
     #[serde(default)]
     pub surface_hooks: Vec<String>,
@@ -132,6 +135,7 @@ impl Default for Config {
                 "separator".into(), "split".into(),
             ],
             surface_timeout_secs: None,
+            surface_conversation_bytes: None,
             surface_hooks: vec![],
         }
     }