feat: add pr diff and checkout (task 0014)

Add the two PR commands that touch content and the local worktree:

- `pr diff <n>` fetches the raw diff from the `.diff` endpoint (forcing a
  text response, which the JSON-defaulting client would otherwise discard),
  truncates at 4000 chars with separate `truncated`/`original_length` fields
  and a prepended `--full` suggestion; `--full` returns the raw diff.
- `pr checkout <n>` reads the PR head branch from the PR fetch and fetches
  `refs/pull/{n}/head` from origin (uniform for same-repo and fork PRs, ADR
  0011), three-cased on local branch state so re-checkout is idempotent and
  divergent local commits fail with `GIT_ERROR` rather than being discarded.

Introduces the `GIT_ERROR` mapping (`runGit`) carrying git's first stderr
line plus a remediation help line, and widens the PR 404 classifier so a
`.diff` path still resolves to `PR_NOT_FOUND`.

.claude/tasks/0014-pr-diff-and-checkout.md

@@ -15,9 +15,29 @@ Output: `checkout: { number, branch, status: "ok" }`.
 
 ## Acceptance criteria
 
-- [ ] `pr diff <n>` outputs the diff, adding `truncated: true` and `original_length` when over 4000 chars plus a `--full` next-step suggestion; `--full` returns the raw diff
-- [ ] `pr checkout <n>` handles all three local-branch cases and re-running it is idempotent
-- [ ] A checked-out branch that has diverged from the PR head fails with `GIT_ERROR` and an explanatory help line, leaving local commits intact
-- [ ] Other git failures (dirty worktree, network) map to `GIT_ERROR` with git's first stderr line
-- [ ] Checkout works for a fork PR whose head repo is not a configured remote (via `refs/pull/{n}/head`)
-- [ ] Tests cover diff truncation boundaries and the three checkout cases (git behavior exercised against a scratch repository, API responses from the fixture server)
+- [x] `pr diff <n>` outputs the diff, adding `truncated: true` and `original_length` when over 4000 chars plus a `--full` next-step suggestion; `--full` returns the raw diff
+- [x] `pr checkout <n>` handles all three local-branch cases and re-running it is idempotent
+- [x] A checked-out branch that has diverged from the PR head fails with `GIT_ERROR` and an explanatory help line, leaving local commits intact
+- [x] Other git failures (dirty worktree, network) map to `GIT_ERROR` with git's first stderr line
+- [x] Checkout works for a fork PR whose head repo is not a configured remote (via `refs/pull/{n}/head`)
+- [x] Tests cover diff truncation boundaries and the three checkout cases (git behavior exercised against a scratch repository, API responses from the fixture server)
+
+## Implementation Notes
+
+The raw diff is fetched through the generated client's `repoDownloadPullDiffOrPatch`, but with `{ format: "text" }` forced per call.
+The `giteaApi` wrapper sets `baseApiParams.format: "json"`, so every response otherwise runs through `response.json()` — which would discard a plain-text `.diff` body and leave `data` null.
+Forcing `text` reads the diff verbatim.
+
+To let the fixture server return a non-JSON diff body, `FixtureServer`'s route gained a `raw?: string` field, served verbatim as `text/plain` (bypassing the `JSON.stringify` the other fields get).
+
+`PULL_PATH` in `src/errors.ts` was widened from `(\d+)(?:\/|$)` to `(\d+)(?:[./]|$)` so a 404 on `/pulls/{n}.diff` still classifies as `PR_NOT_FOUND` rather than falling through to `REPO_NOT_FOUND` — the diff endpoint's number is followed by a `.` suffix rather than a `/` or end-of-path.
+
+Diff truncation is its own `truncateDiff` in `src/diff.ts`, deliberately not reusing `truncateBody`: it signals the cut with separate `truncated`/`original_length` fields (so the diff text stays a verbatim prefix) rather than the inline hint bodies use, and does no body-cleaning.
+
+For the checked-out-and-diverged case, git's `merge --ff-only` prints its `hint:` lines to stderr before the `fatal:` line, so the surfaced `GIT_ERROR` message is that first `hint:` line; the plain-language divergence explanation and remediation live in the help lines, which is where the acceptance criterion's "explanatory help line" is asserted.
+This matches the spec's "carrying git's first stderr line" literally.
+
+`runGit` (the shared git-runner that maps a non-zero exit to `GIT_ERROR` with git's first stderr line) gained an optional `fallbackMessage` argument during the `/review-uncommitted` pass, so the `merge --ff-only` step routes through it instead of re-implementing the enoent/non-zero mapping inline (a Duplicated-Code judgement call the Standards axis raised).
+
+Process note: `/implement` front-loaded the implementation before the test-writer sub-agent authored the tests, so each TDD cycle was green-on-first-run rather than red-first.
+Every test was still authored independently by a `general-purpose` sub-agent from the public CLI interface alone (it never read the implementation source), one behavior at a time.