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/tools/issue_close.go

@@ -0,0 +1,56 @@
+package tools
+
+import (
+	"context"
+	"encoding/json"
+
+	"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/registry"
+)
+
+type IssueClose struct {
+	c *gitea.Client
+	a *allowlist.Allowlist
+}
+
+func NewIssueClose(c *gitea.Client, a *allowlist.Allowlist) *IssueClose {
+	return &IssueClose{c: c, a: a}
+}
+
+func (t *IssueClose) Descriptor() registry.ToolDescriptor {
+	return registry.ToolDescriptor{
+		Name:        "issue_close",
+		Description: "Close an open issue. Reversible via issue_reopen.",
+		InputSchema: json.RawMessage(`{
+			"type":"object",
+			"properties":{
+				"owner":{"type":"string"},
+				"name":{"type":"string"},
+				"number":{"type":"integer","minimum":1}
+			},
+			"required":["owner","name","number"]
+		}`),
+	}
+}
+
+type issueCloseArgs struct {
+	Owner  string `json:"owner"`
+	Name   string `json:"name"`
+	Number int    `json:"number"`
+}
+
+func (t *IssueClose) Call(ctx context.Context, raw json.RawMessage) (json.RawMessage, error) {
+	var args issueCloseArgs
+	if err := parseArgs(raw, &args); err != nil {
+		return nil, err
+	}
+	if err := t.a.Check(args.Owner); err != nil {
+		return nil, err
+	}
+	iss, err := t.c.SetIssueState(ctx, args.Owner, args.Name, args.Number, "closed")
+	if err != nil {
+		return nil, err
+	}
+	return textOK(iss)
+}