feat: add issue_edit tool (#34)

Adds issue_edit, mapping to Gitea's PATCH /repos/{owner}/{repo}/issues/{index},
so an existing issue's title and/or body can be edited through the MCP surface.
Previously the only post-create mutation was issue_comment, which buries
backlinks in the thread instead of the canonical body.

Partial patch via pointer fields (omitempty): omitted fields are left
untouched, an explicit empty string clears a field. Body is sent verbatim —
no identity footer — so repeated edits are idempotent, matching the acceptance
criteria. Registered alongside the other issue tools for tool_search discovery.

Closes #34

internal/gitea/issues_test.go

@@ -76,6 +76,96 @@ func TestGetIssue_NotFound(t *testing.T) {
 	assert.ErrorIs(t, err, gitea.ErrNotFound)
 }
 
+func TestEditIssue(t *testing.T) {
+	var captured []byte
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, http.MethodPatch, r.Method)
+		assert.Equal(t, "/api/v1/repos/o/r/issues/42", r.URL.Path)
+		var err error
+		captured, err = io.ReadAll(r.Body)
+		require.NoError(t, err)
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"number":42,"title":"new title","body":"new body","state":"open","html_url":"http://example.com/issues/42"}`))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+	title, body := "new title", "new body"
+	iss, err := c.EditIssue(context.Background(), "o", "r", 42, gitea.EditIssueArgs{Title: &title, Body: &body})
+	require.NoError(t, err)
+
+	var payload map[string]any
+	require.NoError(t, json.Unmarshal(captured, &payload))
+	assert.Equal(t, "new title", payload["title"])
+	assert.Equal(t, "new body", payload["body"])
+
+	assert.Equal(t, 42, iss.Number)
+	assert.Equal(t, "new title", iss.Title)
+}
+
+// EditIssue must send only the fields explicitly provided — an omitted field
+// (nil pointer) is left untouched server-side.
+func TestEditIssue_PartialPatchOmitsUnsetFields(t *testing.T) {
+	var captured []byte
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		var err error
+		captured, err = io.ReadAll(r.Body)
+		require.NoError(t, err)
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"number":42,"title":"only title","state":"open"}`))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+	title := "only title"
+	_, err := c.EditIssue(context.Background(), "o", "r", 42, gitea.EditIssueArgs{Title: &title})
+	require.NoError(t, err)
+
+	var payload map[string]any
+	require.NoError(t, json.Unmarshal(captured, &payload))
+	assert.Equal(t, "only title", payload["title"])
+	_, hasBody := payload["body"]
+	assert.False(t, hasBody, "body must be omitted when not set")
+}
+
+// An explicit empty-string body clears the field — pointer-to-"" is sent, not omitted.
+func TestEditIssue_EmptyBodyIsSent(t *testing.T) {
+	var captured []byte
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		var err error
+		captured, err = io.ReadAll(r.Body)
+		require.NoError(t, err)
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"number":42,"body":"","state":"open"}`))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+	body := ""
+	_, err := c.EditIssue(context.Background(), "o", "r", 42, gitea.EditIssueArgs{Body: &body})
+	require.NoError(t, err)
+
+	var payload map[string]any
+	require.NoError(t, json.Unmarshal(captured, &payload))
+	val, hasBody := payload["body"]
+	assert.True(t, hasBody, "explicit empty body must be sent so it can clear the field")
+	assert.Equal(t, "", val)
+}
+
+func TestEditIssue_NotFound(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNotFound)
+		_, _ = w.Write([]byte(`{"message":"issue not found"}`))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+	title := "x"
+	_, err := c.EditIssue(context.Background(), "o", "r", 999, gitea.EditIssueArgs{Title: &title})
+	require.Error(t, err)
+	assert.ErrorIs(t, err, gitea.ErrNotFound)
+}
+
 func TestCreateIssueComment(t *testing.T) {
 	var captured []byte
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {