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`.

test/fixture-server.ts

@@ -13,6 +13,11 @@ export interface FixtureRoute {
   body?: unknown;
   /** Name of a JSON file in test/fixtures to serve as the body. */
   fixture?: string;
+  /**
+   * A verbatim, non-JSON response body (e.g. a raw `.diff`). Served as-is with a
+   * `text/plain` content type, bypassing the JSON encoding the other fields get.
+   */
+  raw?: string;
 }
 
 export interface RecordedRequest {
@@ -96,6 +101,14 @@ export async function startFixtureServer(routes: FixtureRoute[]): Promise<Fixtur
         );
         return;
       }
+      if (route.raw !== undefined) {
+        res.writeHead(route.status ?? 200, {
+          "content-type": "text/plain",
+          ...route.headers,
+        });
+        res.end(route.raw);
+        return;
+      }
       res.writeHead(route.status ?? 200, {
         "content-type": "application/json",
         ...route.headers,