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/tools/issue_edit_test.go

@@ -0,0 +1,91 @@
+package tools_test
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"gitea.d-ma.be/mathias/gitea-mcp/internal/allowlist"
+	"gitea.d-ma.be/mathias/gitea-mcp/internal/gitea"
+	"gitea.d-ma.be/mathias/gitea-mcp/internal/tools"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestIssueEditTool(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/mathias/infra/issues/26", 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":26,"title":"new","state":"open","html_url":"http://gitea.example.com/mathias/infra/issues/26"}`))
+	}))
+	defer srv.Close()
+
+	tool := tools.NewIssueEdit(gitea.NewClient(srv.URL, "tok"), allowlist.New([]string{"mathias"}))
+	out, err := tool.Call(context.Background(), json.RawMessage(`{"owner":"mathias","name":"infra","number":26,"title":"new","body":"updated body"}`))
+	require.NoError(t, err)
+
+	var payload map[string]any
+	require.NoError(t, json.Unmarshal(captured, &payload))
+	assert.Equal(t, "new", payload["title"])
+	assert.Equal(t, "updated body", payload["body"])
+
+	assert.Contains(t, string(out), `"number":26`)
+}
+
+// Body must be sent verbatim — no identity footer appended (keeps edits idempotent).
+func TestIssueEditTool_BodyVerbatim(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":26,"state":"open"}`))
+	}))
+	defer srv.Close()
+
+	tool := tools.NewIssueEdit(gitea.NewClient(srv.URL, "tok"), allowlist.New([]string{"mathias"}))
+	_, err := tool.Call(context.Background(), json.RawMessage(`{"owner":"mathias","name":"infra","number":26,"body":"exact text"}`))
+	require.NoError(t, err)
+
+	var payload map[string]any
+	require.NoError(t, json.Unmarshal(captured, &payload))
+	assert.Equal(t, "exact text", payload["body"], "body must be unchanged — no footer")
+	_, hasTitle := payload["title"]
+	assert.False(t, hasTitle, "title must be omitted when not provided")
+}
+
+func TestIssueEditTool_RequiresAField(t *testing.T) {
+	tool := tools.NewIssueEdit(gitea.NewClient("http://unused", ""), allowlist.New([]string{"mathias"}))
+	_, err := tool.Call(context.Background(), json.RawMessage(`{"owner":"mathias","name":"infra","number":26}`))
+	require.Error(t, err)
+	assert.ErrorIs(t, err, gitea.ErrValidation)
+}
+
+func TestIssueEditTool_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()
+
+	tool := tools.NewIssueEdit(gitea.NewClient(srv.URL, "tok"), allowlist.New([]string{"mathias"}))
+	title := "x"
+	body, _ := json.Marshal(map[string]any{"owner": "mathias", "name": "infra", "number": 999, "title": title})
+	_, err := tool.Call(context.Background(), body)
+	require.Error(t, err)
+}
+
+func TestIssueEditAllowlistRejects(t *testing.T) {
+	tool := tools.NewIssueEdit(gitea.NewClient("http://unused", ""), allowlist.New([]string{"mathias"}))
+	_, err := tool.Call(context.Background(), json.RawMessage(`{"owner":"evil","name":"x","number":1,"title":"y"}`))
+	require.Error(t, err)
+}