codex: make abort graceful; force-close MCP on kill/exit

[leeroybrun]

• Problem: Codex “abort” completely killed the current session: it reset internal state and would effectively terminate the session instead of just stopping the current turn. It means that when trying to abort a Codex action, Codex would lose all context of the current session.
• Change:
  • Abort now cancels the in-flight task (AbortController.abort() + reasoning abort) without resetting message queue / permissions / diff state, so the user can immediately send follow-up instructions and continue the same session.
  • Kill/exit uses a hard shutdown path (forceCloseSession) so the Codex MCP transport is torn down and session identifiers are cleared, avoiding leaked child processes and leaving no half-alive state.
• Why: Aligns Codex behavior with Claude behavior: abort = stop current work and keep session alive, kill = archive session and exit.