perf(agent): overlap existing PR checkout with startup

[tatoalo]

Problem

When a cloud task run resumes onto an existing open PR, the agent server checked out the PR branch only after the agent session had started. The checkout sat on the critical path, serializing behind skill-bundle install, native-resume prep, and MCP relay startup, adding wall-clock latency to every run that lands on an existing PR.

Changes

• Overlap the existing-PR checkout with session startup: kick off checkoutExistingPullRequest as a fire-and-forget promise right after waitForRepoReady, then await it only after skill-bundle install, native-resume prep, and the MCP relay server have run in between.
• New packages/agent/src/server/pr-checkout.ts resolves the PR head branch, skips the checkout when that branch is already active, and returns a structured result (already_active / checked_out / failed) so startup never throws on a checkout hiccup.
• On failure, the server logs a warning and falls back to the existing system-prompt instruction that tells the agent to gh pr checkout itself, so a pre-checkout failure can't block session start.

[trunk-io]

😎 Merged successfully - details.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 2a8d038.}

[greptile-apps]

Prompt To Fix All With AI

Fix the following 2 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 2
packages/agent/src/server/pr-checkout.ts:56-57
**Branch Name Hides Remote Mismatch**

For a fork PR whose head branch has the same name as the current local branch, `headRefName` matches even though the local branch belongs to a different remote. This returns `already_active` without checking out the PR head, so the agent can edit or commit against the base branch instead of the existing PR.

### Issue 2 of 2
packages/agent/src/server/agent-server.ts:1508-1517
**Checkout Races Repository Writes**

This starts `gh pr checkout` while skill installation and native-resume preparation write under the same `repositoryPath`. If the target branch replaces or tracks `.posthog/skills` or `sessions`, checkout can conflict with those writes, fail after partial work, or leave startup artifacts associated with the wrong branch.

_{Reviews (1): Last reviewed commit: "perf(agent): overlap existing PR checkou..." | Re-trigger Greptile}

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
packages/agent/src/server/pr-checkout.ts:118-119
**Detached HEAD looks active** When the repository is detached at the PR head commit, `currentHead` equals `prHeadOid`, so this returns `already_active` without switching to the PR branch. The session can then create commits that are not attached to the PR branch, leaving the existing PR unchanged. Verify that HEAD is attached to the expected branch before skipping `gh pr checkout`.

_{Reviews (2): Last reviewed commit: "perf(agent): overlap existing PR checkou..." | Re-trigger Greptile}

[veria-ai]

PR overview

All previously flagged issues have been addressed. No open security concerns remain on this pull request.

Security review

No open security issues remain on this pull request.

Fixed/addressed: 1 · PR risk: 0/10

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
packages/agent/src/server/pr-checkout.ts:157
**Timeout leaves partial checkout.** When the 60-second deadline expires during `gh pr checkout`, aborting the child stops further writes but does not restore the index or working tree. The function then reports `failed` and startup continues, so the agent's fallback checkout can encounter a partially switched or dirty repository and fail again. Preserve the original repository state or restore it after an interrupted checkout before allowing startup to continue.

_{Reviews (3): Last reviewed commit: "perf(agent): overlap existing PR checkou..." | Re-trigger Greptile}

[greptile-apps]

_{Reviews (4): Last reviewed commit: "perf(agent): overlap existing PR checkou..." | Re-trigger Greptile}