fix(#2732): detect crashed subagent sessions with shorter timeout

When a subagent session disappears from the status registry (process
crashed), the main agent was waiting the full stale timeout before
acting. Fix:

- Add sessionGoneTimeoutMs config option (default 60s, vs 30min normal)
- task-poller: use shorter timeout when session is gone from status
- manager: verify session existence when gone, fail crashed tasks
  immediately with descriptive error
- Add legacy-plugin-toast hook for #2823 migration warnings
- Update schema with new config option

assets/oh-my-opencode.schema.json

@@ -4661,6 +4661,10 @@
           "type": "number",
           "minimum": 60000
         },
+        "sessionGoneTimeoutMs": {
+          "type": "number",
+          "minimum": 10000
+        },
         "syncPollTimeoutMs": {
           "type": "number",
           "minimum": 60000