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/pr-diff.test.ts

@@ -0,0 +1,116 @@
+import { afterEach, describe, expect, it } from "vitest";
+import { startFixtureServer, type FixtureServer } from "./fixture-server.js";
+import { runCliTest, testModeEnv } from "./harness.js";
+
+const DIFF_PATH = "/api/v1/repos/testowner/testrepo/pulls/7.diff";
+
+let server: FixtureServer;
+
+afterEach(async () => {
+  await server.close();
+});
+
+describe("pr diff", () => {
+  it("outputs the raw diff verbatim when under the limit, with no truncation fields", async () => {
+    const diff = [
+      "diff --git a/README.md b/README.md",
+      "index 1234567..89abcde 100644",
+      "--- a/README.md",
+      "+++ b/README.md",
+      "@@ -1,3 +1,4 @@",
+      " # Project",
+      " ",
+      "-Old tagline",
+      "+New tagline",
+      "+An extra line",
+      "",
+    ].join("\n");
+
+    server = await startFixtureServer([
+      { method: "GET", path: DIFF_PATH, raw: diff },
+    ]);
+
+    const { stdout, exitCode } = await runCliTest(["pr", "diff", "7"], {
+      env: testModeEnv(server.url),
+    });
+
+    expect(exitCode).toBe(0);
+    expect(stdout).toContain("pr_diff:");
+    expect(stdout).toContain("number: 7");
+    expect(stdout).toContain("+New tagline");
+    expect(stdout).not.toContain("truncated:");
+    expect(stdout).not.toContain("original_length:");
+  });
+
+  it("truncates a diff over 4000 chars to the first 4000 and reports the original length", async () => {
+    const diff = "x".repeat(4100);
+
+    server = await startFixtureServer([
+      { method: "GET", path: DIFF_PATH, raw: diff },
+    ]);
+
+    const { stdout, exitCode } = await runCliTest(["pr", "diff", "7"], {
+      env: testModeEnv(server.url),
+    });
+
+    expect(exitCode).toBe(0);
+    expect(stdout).toContain("truncated: true");
+    expect(stdout).toContain("original_length: 4100");
+    expect(stdout).toContain("Run `gitea-axi pr diff 7");
+    expect(stdout).toContain("--full");
+    expect(stdout).toContain("to see the complete diff");
+    expect(stdout).toContain("x".repeat(4000));
+    expect(stdout).not.toContain("x".repeat(4001));
+  });
+
+  it("passes a diff of exactly 4000 chars through untouched", async () => {
+    const diff = "y".repeat(4000);
+
+    server = await startFixtureServer([
+      { method: "GET", path: DIFF_PATH, raw: diff },
+    ]);
+
+    const { stdout, exitCode } = await runCliTest(["pr", "diff", "7"], {
+      env: testModeEnv(server.url),
+    });
+
+    expect(exitCode).toBe(0);
+    expect(stdout).not.toContain("truncated:");
+    expect(stdout).not.toContain("original_length:");
+  });
+
+  it("returns the complete diff and suppresses truncation with --full", async () => {
+    const diff = "z".repeat(4100);
+
+    server = await startFixtureServer([
+      { method: "GET", path: DIFF_PATH, raw: diff },
+    ]);
+
+    const { stdout, exitCode } = await runCliTest(["pr", "diff", "7", "--full"], {
+      env: testModeEnv(server.url),
+    });
+
+    expect(exitCode).toBe(0);
+    expect(stdout).toContain("z".repeat(4100));
+    expect(stdout).not.toContain("truncated:");
+    expect(stdout).not.toContain("original_length:");
+  });
+
+  it("reports a nonexistent pull request as PR_NOT_FOUND with exit 1", async () => {
+    server = await startFixtureServer([
+      {
+        method: "GET",
+        path: "/api/v1/repos/testowner/testrepo/pulls/999.diff",
+        status: 404,
+        body: { message: "Not Found" },
+      },
+    ]);
+
+    const { stdout, exitCode } = await runCliTest(["pr", "diff", "999"], {
+      env: testModeEnv(server.url),
+    });
+
+    expect(exitCode).toBe(1);
+    expect(stdout).toContain("code: PR_NOT_FOUND");
+  });
+});