feat: implement POST /pulls merge handler with merge button in PR detail

Add MergePull HTTP handler that calls the existing client.MergePull()
method, register the POST /pulls/{owner}/{repo}/{index}/merge route,
and add a merge button/form to the PR detail template gated on
Mergeable and open state. Supports merge, rebase, and squash styles.

- Handler returns HTMX fragment on HX-Request, redirect otherwise
- Error path returns inline error fragment for HTMX requests
- Add mock merge endpoint and 3 integration tests
- Merge button only shows when PR is mergeable and open

Closes leeworks-agents/gitea-mobile#229

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/handlers/handlers.go

@@ -56,6 +56,7 @@ func (h *Handler) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("GET /pulls/{owner}/{repo}/{index}", h.PullDetail)
 	mux.HandleFunc("POST /pulls/{owner}/{repo}/{index}/review", h.SubmitReview)
 	mux.HandleFunc("POST /pulls/{owner}/{repo}/{index}/state", h.SetPullState)
+	mux.HandleFunc("POST /pulls/{owner}/{repo}/{index}/merge", h.MergePull)
 
 	// Settings (handled separately for auth bypass).
 	settingsHandler := &SettingsHandler{
@@ -1102,3 +1103,49 @@ func (h *Handler) SubmitReview(w http.ResponseWriter, r *http.Request) {
 
 	http.Redirect(w, r, fmt.Sprintf("/pulls/%s/%s/%d", owner, repo, index), http.StatusSeeOther)
 }
+
+// MergePull handles POST /pulls/{owner}/{repo}/{index}/merge.
+func (h *Handler) MergePull(w http.ResponseWriter, r *http.Request) {
+	token := getToken(r)
+	owner := r.PathValue("owner")
+	repo := r.PathValue("repo")
+	indexStr := r.PathValue("index")
+
+	index, err := strconv.ParseInt(indexStr, 10, 64)
+	if err != nil {
+		http.Error(w, "invalid PR index", http.StatusBadRequest)
+		return
+	}
+
+	if err := r.ParseForm(); err != nil {
+		http.Error(w, "bad request", http.StatusBadRequest)
+		return
+	}
+
+	style := r.FormValue("Do")
+	if style == "" {
+		style = "merge"
+	}
+	title := r.FormValue("merge_title")
+	message := r.FormValue("merge_message")
+
+	if err := h.Client.MergePull(r.Context(), token, owner, repo, index, style, title, message); err != nil {
+		slog.Error("failed to merge pull request", "error", err, "owner", owner, "repo", repo, "index", index)
+		if isHTMX(r) {
+			w.Header().Set("Content-Type", "text/html; charset=utf-8")
+			w.WriteHeader(http.StatusInternalServerError)
+			fmt.Fprint(w, `<span style="color:#f85149">Failed to merge pull request.</span>`)
+			return
+		}
+		http.Error(w, "failed to merge pull request", http.StatusInternalServerError)
+		return
+	}
+
+	if isHTMX(r) {
+		w.Header().Set("Content-Type", "text/html; charset=utf-8")
+		fmt.Fprint(w, `<span style="color:#a371f7;font-weight:bold">Merged</span>`)
+		return
+	}
+
+	http.Redirect(w, r, fmt.Sprintf("/pulls/%s/%s/%d", owner, repo, index), http.StatusSeeOther)
+}

internal/handlers/integration_test.go

@@ -185,6 +185,11 @@ func mockGiteaAPI(t *testing.T) *httptest.Server {
 		json.NewEncoder(w).Encode(map[string]interface{}{"id": 1, "state": "APPROVED"})
 	})
 
+	// POST /api/v1/repos/{owner}/{repo}/pulls/{index}/merge — merge PR.
+	mux.HandleFunc("POST /api/v1/repos/{owner}/{repo}/pulls/{index}/merge", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	})
+
 	// POST /api/v1/markdown — render markdown.
 	mux.HandleFunc("POST /api/v1/markdown", func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Set("Content-Type", "text/html")
@@ -917,6 +922,67 @@ func TestIntegration_SubmitReview_RequestChanges(t *testing.T) {
 	}
 }
 
+// --- Issue #229: Integration tests for POST /pulls/{owner}/{repo}/{index}/merge ---
+
+func TestIntegration_MergePull_Valid(t *testing.T) {
+	h, srv := newTestHandlerWithMock(t)
+	defer srv.Close()
+
+	mux := http.NewServeMux()
+	mux.HandleFunc("POST /pulls/{owner}/{repo}/{index}/merge", h.MergePull)
+
+	form := url.Values{"Do": {"merge"}}
+	req := reqWithToken(http.MethodPost, "/pulls/test-org/repo1/1/merge", form.Encode())
+	w := httptest.NewRecorder()
+
+	mux.ServeHTTP(w, req)
+
+	if w.Code != http.StatusSeeOther {
+		t.Errorf("status = %d, want %d", w.Code, http.StatusSeeOther)
+	}
+}
+
+func TestIntegration_MergePull_HTMX(t *testing.T) {
+	h, srv := newTestHandlerWithMock(t)
+	defer srv.Close()
+
+	mux := http.NewServeMux()
+	mux.HandleFunc("POST /pulls/{owner}/{repo}/{index}/merge", h.MergePull)
+
+	form := url.Values{"Do": {"squash"}}
+	req := reqWithToken(http.MethodPost, "/pulls/test-org/repo1/1/merge", form.Encode())
+	req.Header.Set("HX-Request", "true")
+	w := httptest.NewRecorder()
+
+	mux.ServeHTTP(w, req)
+
+	if w.Code != http.StatusOK {
+		t.Errorf("status = %d, want %d", w.Code, http.StatusOK)
+	}
+	body := w.Body.String()
+	if !contains(body, "Merged") {
+		t.Errorf("expected 'Merged' in HTMX response, got: %s", body)
+	}
+}
+
+func TestIntegration_MergePull_DefaultStyle(t *testing.T) {
+	h, srv := newTestHandlerWithMock(t)
+	defer srv.Close()
+
+	mux := http.NewServeMux()
+	mux.HandleFunc("POST /pulls/{owner}/{repo}/{index}/merge", h.MergePull)
+
+	// No "Do" form value — should default to "merge".
+	req := reqWithToken(http.MethodPost, "/pulls/test-org/repo1/1/merge", "")
+	w := httptest.NewRecorder()
+
+	mux.ServeHTTP(w, req)
+
+	if w.Code != http.StatusSeeOther {
+		t.Errorf("status = %d, want %d", w.Code, http.StatusSeeOther)
+	}
+}
+
 // --- Issue #110: Integration tests for POST /issues/{owner}/{repo}/{index}/labels ---
 
 func TestIntegration_ApplyLabels_Valid(t *testing.T) {

internal/templates/pull_detail.html

@@ -33,6 +33,23 @@
   {{end}}
 </div>
 
+{{if and .Pull.Mergeable (eq .Pull.State "open")}}
+<div class="card" style="margin-top:1rem;" id="merge-section">
+  <h2>Merge Pull Request</h2>
+  <form hx-post="/pulls/{{.Pull.RepoOwner}}/{{.Pull.RepoName}}/{{.Pull.Number}}/merge" hx-target="#merge-section" hx-swap="innerHTML">
+    <div class="form-group">
+      <label for="merge-style">Merge Style</label>
+      <select id="merge-style" name="Do">
+        <option value="merge">Merge Commit</option>
+        <option value="rebase">Rebase</option>
+        <option value="squash">Squash</option>
+      </select>
+    </div>
+    <button type="submit" class="btn btn-primary" style="background-color:var(--accent-purple,#a371f7);">Merge PR</button>
+  </form>
+</div>
+{{end}}
+
 <div class="card" style="margin-top:1rem;">
   <h2>Submit Review</h2>
   <form hx-post="/pulls/{{.Pull.RepoOwner}}/{{.Pull.RepoName}}/{{.Pull.Number}}/review" hx-swap="outerHTML">