feat(hooks): emit session:end lifecycle event

Adds session:end hook emitted before session:reset, giving hook authors
a clean teardown signal to persist data or clean up resources before the
session is destroyed.

Based on PR #1564 by bayrakdarerdem (session:start portion was already
on main).

gateway/hooks.py

@@ -8,8 +8,9 @@ Hooks are discovered from ~/.hermes/hooks/ directories, each containing:
 
 Events:
   - gateway:startup     -- Gateway process starts
-  - session:start       -- New session created
-  - session:reset       -- User ran /new or /reset
+  - session:start       -- New session created (first message of a new session)
+  - session:end         -- Session ends (user ran /new or /reset)
+  - session:reset       -- Session reset completed (new session entry created)
   - agent:start         -- Agent begins processing a message
   - agent:step          -- Each turn in the tool-calling loop
   - agent:end           -- Agent finishes processing

gateway/run.py

@@ -2178,7 +2178,14 @@ class GatewayRunner:
         
         # Reset the session
         new_entry = self.session_store.reset_session(session_key)
-        
+
+        # Emit session:end hook (session is ending)
+        await self.hooks.emit("session:end", {
+            "platform": source.platform.value if source.platform else "",
+            "user_id": source.user_id,
+            "session_key": session_key,
+        })
+
         # Emit session:reset hook
         await self.hooks.emit("session:reset", {
             "platform": source.platform.value if source.platform else "",