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"`