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/session.rs

@@ -32,9 +32,8 @@ impl Session {
         self.state_dir.join(format!("{}-{}", prefix, self.session_id))
     }
 
-    /// Load from POC_SESSION_ID environment variable
-    pub fn from_env() -> Option<Self> {
-        let session_id = std::env::var("POC_SESSION_ID").ok()?;
+    /// Load from a session ID string
+    pub fn from_id(session_id: String) -> Option<Self> {
         if session_id.is_empty() { return None; }
         let state_dir = PathBuf::from("/tmp/claude-memory-search");
         Some(Session {
@@ -45,6 +44,11 @@ impl Session {
         })
     }
 
+    /// Load from POC_SESSION_ID environment variable
+    pub fn from_env() -> Option<Self> {
+        Self::from_id(std::env::var("POC_SESSION_ID").ok()?)
+    }
+
     /// Get the seen set for this session
     pub fn seen(&self) -> HashSet<String> {
         super::hippocampus::memory_search::load_seen(&self.state_dir, &self.session_id)