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.go

@@ -9,16 +9,16 @@ import (
 )
 
 type Issue struct {
-	Number    int      `json:"number"`
-	Title     string   `json:"title"`
-	Body      string   `json:"body"`
-	HTMLURL   string   `json:"html_url"`
-	State     string   `json:"state"`
-	CreatedAt string   `json:"created_at"`
-	UpdatedAt string   `json:"updated_at"`
-	Labels    []Label  `json:"labels"`
-	Assignees []User   `json:"assignees"`
-	Comments  int      `json:"comments"`
+	Number    int     `json:"number"`
+	Title     string  `json:"title"`
+	Body      string  `json:"body"`
+	HTMLURL   string  `json:"html_url"`
+	State     string  `json:"state"`
+	CreatedAt string  `json:"created_at"`
+	UpdatedAt string  `json:"updated_at"`
+	Labels    []Label `json:"labels"`
+	Assignees []User  `json:"assignees"`
+	Comments  int     `json:"comments"`
 }
 
 type Label struct {
@@ -140,6 +140,37 @@ func (c *Client) SetIssueState(ctx context.Context, owner, repo string, number i
 	return &iss, nil
 }
 
+// EditIssueArgs uses pointers so omitempty distinguishes "not set" (nil,
+// left untouched) from an explicit empty string (clears the field). Maps to
+// Gitea's PATCH /repos/{owner}/{repo}/issues/{index}.
+type EditIssueArgs struct {
+	Title *string `json:"title,omitempty"`
+	Body  *string `json:"body,omitempty"`
+}
+
+// EditIssue patches an issue's title and/or body. Only fields set in args are
+// sent, so omitted fields are left as-is server-side. Body is sent verbatim —
+// no identity footer — so repeated edits are idempotent.
+func (c *Client) EditIssue(ctx context.Context, owner, repo string, number int, args EditIssueArgs) (*Issue, error) {
+	p := fmt.Sprintf("/api/v1/repos/%s/%s/issues/%d", owner, repo, number)
+	payload, err := json.Marshal(args)
+	if err != nil {
+		return nil, err
+	}
+	body, status, err := c.PatchJSON(ctx, p, payload)
+	if err != nil {
+		return nil, err
+	}
+	if err := MapStatus(status, body); err != nil {
+		return nil, err
+	}
+	var iss Issue
+	if err := json.Unmarshal(body, &iss); err != nil {
+		return nil, err
+	}
+	return &iss, nil
+}
+
 type IssueComment struct {
 	ID        int64  `json:"id"`
 	Body      string `json:"body"`

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) {