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`.

src/errors.ts

@@ -55,7 +55,9 @@ function pathname(url: string): string {
 }
 
 const ISSUE_PATH = /\/repos\/[^/]+\/[^/]+\/issues\/(\d+)(?:\/|$)/;
-const PULL_PATH = /\/repos\/[^/]+\/[^/]+\/pulls\/(\d+)(?:\/|$)/;
+// The trailing `.` case covers the `.diff`/`.patch` download paths, whose PR
+// number is followed by a suffix rather than a `/` or the end of the path.
+const PULL_PATH = /\/repos\/[^/]+\/[^/]+\/pulls\/(\d+)(?:[./]|$)/;
 const REPO_PATH = /\/repos\/([^/]+)\/([^/]+)(?:\/|$)/;
 
 function classify404(response: HttpResponseLike): AxiError {