feat: issue_close + issue_reopen tools (#30)

Adds two MCP tools that PATCH /api/v1/repos/{owner}/{name}/issues/{number}
with {"state":"closed"} or {"state":"open"}. Both use a shared
SetIssueState helper on the gitea client.

- internal/gitea/issues.go: SetIssueState method using the existing
  PatchJSON + MapStatus + json.Unmarshal pattern from GetIssue.
- internal/tools/issue_close.go: IssueClose tool. owner+name+number
  args. Owner allowlist enforced. Returns the updated issue. Reversible
  via issue_reopen, classified LOW risk.
- internal/tools/issue_reopen.go: mirror of IssueClose with
  state="open". Same risk profile.
- Registered both tools in cmd/gitea-mcp/main.go.
- Tests for both: success (asserts PATCH method, path, body), 404,
  and allowlist rejection — same shape as issue_get_test.go.

Closes #30

internal/gitea/issues.go

@@ -72,6 +72,28 @@ func (c *Client) CreateIssue(ctx context.Context, owner, repo string, args Creat
 	return &iss, nil
 }
 
+// SetIssueState flips an issue between "open" and "closed" via PATCH.
+// Gitea uses the same endpoint for both transitions.
+func (c *Client) SetIssueState(ctx context.Context, owner, repo string, number int, state string) (*Issue, error) {
+	p := fmt.Sprintf("/api/v1/repos/%s/%s/issues/%d", owner, repo, number)
+	payload, err := json.Marshal(map[string]string{"state": state})
+	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"`