feat(agent): resume codex cloud threads natively on restarts

[k11kirky]

Problem

The Codex adapter already supports native resume (thread/resume via ACP resumeSession), but the cloud agent-server never used it — prepareNativeResume hard-returned null for anything non-Claude. Every Codex cloud resume therefore started a fresh thread and injected the prior conversation as a hidden Markdown blob, losing reasoning context, compaction state, structured tool history, and prompt-cache continuity.

Change

• prepareNativeResume now handles Codex: it finds the prior session id from the run log (for Codex the ACP sessionId is the codex thread id) and checks whether the thread's rollout file still exists under CODEX_HOME/sessions/ via the new hasCodexThreadState helper. If so, the server calls resumeSession and the codex app-server restores the full thread natively — reasoning items, compaction state and all — with the continuation prompt riding the resumed thread.
• The rollout only survives a snapshot (warm) sandbox restart, and there is no faithful cold-hydration equivalent (rollouts contain encrypted reasoning we can't reconstruct from logs), so a cold sandbox keeps today's Markdown-summary fallback. Warm resumes also skip the redundant git-checkpoint restore, matching Claude's warm path.
• Safety net: if resumeSession fails at boot (e.g. a corrupt rollout), the server logs a warning and falls back to a fresh session with the summary resume instead of failing the run. Applies to Claude too, which previously had no fallback there.

Testing

• 5 new unit tests for the rollout lookup, 3 new agent-server tests covering warm-resume, thread-gone, and no-session-id paths.
• Full @posthog/agent suite: 1093 passed; remaining failures are pre-existing on main and sandbox-environment artifacts (ambient GH_TOKEN, chmod no-op as root).
• tsc --noEmit and Biome clean.

Follow-up (not in this PR)

• Desktop→cloud handoffs could ship the rollout file into the sandbox/checkpoint to make cold restores native too — needs a transport for CODEX_HOME state.
• "Can this session be natively resumed" could become an adapter capability instead of per-adapter branching in the server, but Claude's check isn't adapter-self-contained (hydration needs the server's API client), so that refactor is deferred.

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 30042b7.}

[greptile-apps]

Comments Outside Diff (1)

1. packages/agent/src/server/agent-server.test.ts, line 1840-1845 (link)

   Two null-expectation cases are good candidates for it.each

   The "thread state gone" and "no prior session id" tests both call prepare(codexServer(…)) and assert toBeNull(). They differ only in the sessionId passed to codexServer. An it.each with [THREAD_ID, "no rollout present"] / [null, "no session id"] pairs would reduce duplication and align with the repo's parameterised-test preference.

   Context Used: Do not attempt to comment on incorrect alphabetica... (source)

   Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

_{Reviews (1): Last reviewed commit: "feat(agent): resume codex cloud threads ..." | Re-trigger Greptile}