feat(tools): pr_files_diff with caps

Returns per-file unified diff for a PR, capped at 20KB/file and 200KB
total response. Files exceeding per-file cap report truncated+omitted_lines;
files that would push the response over 200KB go to omitted_files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/gitea/pulls_test.go

@@ -93,3 +93,46 @@ func TestGetPullRequest(t *testing.T) {
 	assert.Equal(t, "open", pr.State)
 	assert.True(t, pr.Draft)
 }
+
+func TestGetPullRequestFiles(t *testing.T) {
+	filesJSON := `[
+		{"filename":"main.go","status":"modified","additions":10,"deletions":5},
+		{"filename":"README.md","status":"added","additions":20,"deletions":0}
+	]`
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, "/api/v1/repos/o/r/pulls/42/files", r.URL.Path)
+		assert.Equal(t, http.MethodGet, r.Method)
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(filesJSON))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+	files, err := c.GetPullRequestFiles(context.Background(), "o", "r", 42)
+	require.NoError(t, err)
+	require.Len(t, files, 2)
+	assert.Equal(t, "main.go", files[0].Filename)
+	assert.Equal(t, "modified", files[0].Status)
+	assert.Equal(t, 10, files[0].Additions)
+	assert.Equal(t, 5, files[0].Deletions)
+	assert.Equal(t, "README.md", files[1].Filename)
+	assert.Equal(t, "added", files[1].Status)
+	assert.Equal(t, 20, files[1].Additions)
+	assert.Equal(t, 0, files[1].Deletions)
+}
+
+func TestGetPullRequestDiff(t *testing.T) {
+	rawDiff := "diff --git a/main.go b/main.go\n--- a/main.go\n+++ b/main.go\n@@ -1,2 +1,3 @@\n+package main\n"
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, "/api/v1/repos/o/r/pulls/42.diff", r.URL.Path)
+		assert.Equal(t, http.MethodGet, r.Method)
+		w.Header().Set("Content-Type", "text/plain")
+		_, _ = w.Write([]byte(rawDiff))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+	diff, err := c.GetPullRequestDiff(context.Background(), "o", "r", 42)
+	require.NoError(t, err)
+	assert.Equal(t, []byte(rawDiff), diff)
+}