feat(memory): add session_id isolation to Memory trait (#530)

* feat(memory): add session_id isolation to Memory trait

Add optional session_id parameter to store(), recall(), and list()
methods across the Memory trait and all four backends (sqlite, markdown,
lucid, none). This enables per-session memory isolation so different
agent sessions cannot cross-read each other's stored memories.

Changes:
- traits.rs: Add session_id: Option<&str> to store/recall/list
- sqlite.rs: Schema migration (ALTER TABLE ADD COLUMN session_id),
  index, persist/filter by session_id in all query paths
- markdown.rs, lucid.rs, none.rs: Updated signatures
- All callers pass None for backward compatibility
- 5 new tests: session-filtered recall, cross-session isolation,
  session-filtered list, no-filter returns all, migration idempotency

Closes #518

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix(channels): fix discord _channel_id typo and lark missing reply_to

Pre-existing compilation errors on main after reply_to was added to
ChannelMessage: discord.rs used _channel_id (underscore prefix) but
referenced channel_id, and lark.rs was missing the reply_to field.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/agent/agent.rs

@@ -389,7 +389,7 @@ impl Agent {
         if self.auto_save {
             let _ = self
                 .memory
-                .store("user_msg", user_message, MemoryCategory::Conversation)
+                .store("user_msg", user_message, MemoryCategory::Conversation, None)
                 .await;
         }
 
@@ -448,7 +448,7 @@ impl Agent {
                     let summary = truncate_with_ellipsis(&final_text, 100);
                     let _ = self
                         .memory
-                        .store("assistant_resp", &summary, MemoryCategory::Daily)
+                        .store("assistant_resp", &summary, MemoryCategory::Daily, None)
                         .await;
                 }