Improve stale session forensics and question recovery

[dmoliveira]

Summary

• prevent fresh question tool lifecycles from being prematurely recovered during idle session recovery while still cleaning up tracked question state on reply, completion, and error paths
• expose richer /session doctor stale forensics with derived cause codes/summaries for stuck and generic stale findings, and normalize the output for easier debugging
• ignore generated gateway-core tarballs and extend selftests to cover the new stale recovery and forensic reporting behavior

Validation

• python3 -m py_compile scripts/session_command.py scripts/selftest.py
• python3 scripts/selftest.py
• npm --prefix plugin/gateway-core run build
• node --test plugin/gateway-core/test/session-recovery-hook.test.mjs
• npm --prefix plugin/gateway-core run lint