refactor(review): git-native PR inspection (replaces prompt-hydrated patch text, per #68)

[stainlu]

Following #68's close — confirming the replacement design before opening code so it matches the intended direction.

Plan:

• Keep the target checkout on main (preserve the read-only invariant — do not materialize the PR as the working tree).
• Reuse the existing fetchSourcePullRequestHead primitive (src/repair/source-pr-checkout.ts) to fetch refs/pull/<n>/head into a read-only refs/remotes/clawsweeper/... ref — no new parallel helper.
• Instruct Codex in prompts/review-item.md to inspect via git diff origin/main...<ref> and git show <ref>:path.
• Slim compactPullFilesForPrompt to file metadata only, dropping patch bodies. Verified .patch is consumed only by the prompt builder; repair/automerge gates run on the decision JSON + live git, so nothing downstream breaks.

Open question I want to get right: the main checkout is --filter=blob:none, so I'd pre-warm the PR diff blobs deterministically under ClawSweeper's control rather than let Codex trigger lazy promisor fetches inside the sandbox.

Net effect: deletes the truncation layer entirely, simpler core bot. @steipete does this match the direction from #68? I'll scope it as refactor(review): off fresh main.

[clawsweeper]

Codex review: keeping this open for maintainer follow-up; there is still a little grit to resolve.

Workflow note: Future ClawSweeper reviews update this same comment in place.

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Summary
Keep open: current main still hydrates PR file patch text into the review context, while the related maintainer discussion supports the git-native direction without having landed the replacement. This is an internal review-lane refactor with one remaining design choice, so it needs maintainer confirmation rather than cleanup closure.

Reproducibility: not applicable. This issue is a refactor/design proposal, not a bug report with a runtime reproduction path. Source inspection is enough to show the requested replacement is not yet present on current main.

Ways to help us reproduce this

• Add a screenshot or short recording showing the behavior.
• Add expected vs actual behavior.
• Include redacted logs or terminal output.

Next step
A maintainer should confirm the blob pre-warming and review-prompt contract before this becomes an implementation task.

Review details

Best possible solution:

Land one focused refactor after maintainer confirmation: reuse the existing PR-head fetch helper, deterministically prepare the diff data, update the review prompt, and keep repair/automerge gates tied to decision JSON plus live git.

Do we have a high-confidence way to reproduce the issue?

Not applicable: this issue is a refactor/design proposal, not a bug report with a runtime reproduction path. Source inspection is enough to show the requested replacement is not yet present on current main.

Is this the best way to solve the issue?

Unclear: the direction matches the related maintainer comment, but the issue explicitly raises a design question around deterministic blob pre-warming. The narrowest maintainable solution should be confirmed before implementation.

Label changes:

• add P3: This is a low-risk internal review-lane refactor and prompt-efficiency cleanup, not a user-facing regression.
• add issue-rating: 🌊 off-meta tidepool: Current issue advisory state selects this label.
• add clawsweeper:no-new-fix-pr: Current issue advisory state selects this label.
• add clawsweeper:needs-maintainer-review: Current issue advisory state selects this label.
• add clawsweeper:needs-product-decision: Current issue advisory state selects this label.

Label justifications:

• P3: This is a low-risk internal review-lane refactor and prompt-efficiency cleanup, not a user-facing regression.

Acceptance criteria:

• pnpm run check

What I checked:

• PR file context still includes patch bodies: The current PR file compactor still returns patch: truncateText(file.patch, 2000), so the requested metadata-only PR file hydration has not landed. (src/clawsweeper.ts:2876, 43cfca13e47e)
• Review lane still hydrates PR files through GitHub context: The review context builder still calls the pull files API, maps the result through compactPullFile, and records pull-file truncation counts instead of relying on a fetched PR ref. (src/clawsweeper.ts:4335, 43cfca13e47e)
• Existing source PR fetch helper is available: The repair helper already fetches refs/pull/<n>/head into refs/remotes/clawsweeper/source-pr-<n>, matching the primitive this issue proposes to reuse. (src/repair/source-pr-checkout.ts:83, 43cfca13e47e)
• Review prompt has not been switched to the proposed git-native instructions: A source search found no prompt instruction for git diff origin/main...<ref> or git show <ref>:path; the current prompt only lists generic read-only inspection commands. (prompts/review-item.md:13, 43cfca13e47e)
• Related pull request was closed with maintainer direction: The related closed pull request perf: budget review prompt file patches #68 has a maintainer comment saying the review path is moving toward Codex inspecting the PR branch directly, but the pull request was closed unmerged.
• No newer canonical implementation PR was found: Searches for the replacement direction found this issue and the closed unmerged related pull request, but no open or merged implementation PR that supersedes the remaining work.

Likely related people:

• Tak Hoffman: Git blame for the current PR file compactor, review prompt, and source PR checkout helper points to the visible current-main import commit for these paths. (role: current implementation author; confidence: medium; commits: a54b3459cda9; files: src/clawsweeper.ts, prompts/review-item.md, src/repair/source-pr-checkout.ts)
• steipete: Commented on perf: budget review prompt file patches #68 that the review path should move toward Codex inspecting the PR branch directly, which is the direction this issue is asking to confirm. (role: related design commenter; confidence: medium; files: src/clawsweeper.ts, prompts/review-item.md)
• stainlu: Authored this issue and the related unmerged patch-budgeting pull request that prompted the replacement-design discussion. (role: related proposal author; confidence: medium; commits: 20f5662b1fab; files: src/clawsweeper.ts, test/clawsweeper.test.ts)

Remaining risk / open question:

• The remaining open question is how the review lane should pre-warm partial-clone PR diff blobs without letting Codex trigger uncontrolled lazy fetches.

Codex review notes: model gpt-5.5, reasoning high; reviewed against 43cfca13e47e.