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/diff.ts

@@ -0,0 +1,58 @@
+import type { GiteaClient } from "./client.js";
+import type { RepoContext } from "./context.js";
+import { classifyHttpError } from "./errors.js";
+
+/** The raw-diff truncation limit, distinct from the body/comment limits. */
+export const DIFF_TRUNCATE_LIMIT = 4000;
+
+/**
+ * A rendered pull-request diff. `truncated`/`original_length` are present only
+ * when the diff was cut short — signalled as separate fields rather than an
+ * inline hint, so the diff text itself stays a verbatim (if partial) prefix.
+ */
+export interface DiffResult {
+  diff: string;
+  truncated?: true;
+  original_length?: number;
+}
+
+/**
+ * Fetch a pull request's raw unified diff from `GET /pulls/{index}.diff`. The
+ * generated client defaults every response to JSON parsing (its baseApiParams
+ * set `format: "json"`), which would discard a plain-text diff body — so `text`
+ * is forced per call to read the response verbatim.
+ */
+export async function fetchPullDiff(
+  api: GiteaClient,
+  context: RepoContext,
+  number: number,
+): Promise<string> {
+  try {
+    const response = await api.repos.repoDownloadPullDiffOrPatch(
+      context.owner,
+      context.name,
+      number,
+      "diff",
+      undefined,
+      { format: "text" },
+    );
+    return response.data ?? "";
+  } catch (error) {
+    throw classifyHttpError(error);
+  }
+}
+
+/**
+ * Truncate a diff to {@link DIFF_TRUNCATE_LIMIT}, reporting the original length
+ * so the caller can offer `--full`. `full` returns the raw diff untouched.
+ */
+export function truncateDiff(diff: string, full: boolean): DiffResult {
+  if (full || diff.length <= DIFF_TRUNCATE_LIMIT) {
+    return { diff };
+  }
+  return {
+    diff: diff.slice(0, DIFF_TRUNCATE_LIMIT),
+    truncated: true,
+    original_length: diff.length,
+  };
+}