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

cmd/gitea-mcp/main.go

@@ -66,6 +66,8 @@ func main() {
 	reg.Register(tools.NewRepoTree(giteaClient, ownerAllow))
 	reg.Register(tools.NewRepoTopicsUpdate(giteaClient, ownerAllow))
 	reg.Register(tools.NewIssueGet(giteaClient, ownerAllow))
+	reg.Register(tools.NewIssueClose(giteaClient, ownerAllow))
+	reg.Register(tools.NewIssueReopen(giteaClient, ownerAllow))
 	reg.Register(tools.NewReleaseCreate(giteaClient, ownerAllow))
 	reg.Register(tools.NewRepoDelete(giteaClient, ownerAllow))