feat: Harden CodexAcpAgent session state and permission handling

[charlesvien]

Changes

1. Initialize sessionState and codexConnection eagerly in the constructor instead of using ! assertions
2. Pass permissionMode, taskRunId and taskId from meta into createSessionState for all session creation paths (load, new, fork)
3. Fix double-counting of token usage by relying on sessionUpdate notifications instead of also adding response.usage
4. Remove redundant cancel() override, use inherited interrupt() and base class cancellation state
5. Use POSTHOG_NOTIFICATIONS.USAGE_UPDATE constant instead of hardcoded string
6. Escape newlines in system instructions passed to codex CLI args
7. Abort the session's AbortController on closeSession for proper async cleanup

How did you test this?

Manually

[charlesvien]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat: Harden CodexAcpAgent session state and permission handling #1487 👈 (View in Graphite)
• feat: give codex permissions support #1483
• feat: give codex system prompt via personality #1482
• feat: clean up session reloadin #1481
• feat: update codex adapter and migrate to base acp prxy for agent #1480
• feat: codex 5.4 default and pass to model #1479
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