feat(hook): add spawn_id correlation to amc-hook

Read AMC_SPAWN_ID env var and include spawn_id in session JSON when
present. This enables deterministic spawn correlation: the server
generates a UUID, passes it via env to the spawned agent, and then
polls for a session file containing that specific spawn_id.

Closes: bd-1zy

bin/amc-hook

@@ -249,6 +249,11 @@ def main():
             "zellij_pane": os.environ.get("ZELLIJ_PANE_ID", ""),
         }
 
+        # Include spawn_id if present in environment (for spawn correlation)
+        spawn_id = os.environ.get("AMC_SPAWN_ID")
+        if spawn_id:
+            state["spawn_id"] = spawn_id
+
         # Turn timing: track working time from user prompt to completion
         if event == "UserPromptSubmit":
             # New turn starting - reset turn timing