fix: normalize Forgejo PR/issue state to uppercase

Forgejo REST API returns state as lowercase ("closed"/"open") but
branch.ts compares against GraphQL-style uppercase ("CLOSED"/"MERGED").
This caused @claude on closed/merged PRs to take the open-PR checkout
path, failing with "fatal: invalid reference: refs/pull/N/head".

Normalize state to uppercase in the Forgejo data fetcher, matching the
existing pattern used for review state (line 153).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/forgejo-data-fetcher.test.ts

@@ -211,7 +211,7 @@ describe("fetchForgejoData", () => {
       expect(pr.lastEditedAt).toBeUndefined();
       expect(pr.additions).toBe(42);
       expect(pr.deletions).toBe(15);
-      expect(pr.state).toBe("open");
+      expect(pr.state).toBe("OPEN");
     });
 
     it("should map PR labels correctly", async () => {
@@ -455,7 +455,7 @@ describe("fetchForgejoData", () => {
       expect(issue.createdAt).toBe("2024-01-10T08:00:00Z");
       expect(issue.updatedAt).toBe("2024-01-14T09:00:00Z");
       expect(issue.lastEditedAt).toBeUndefined();
-      expect(issue.state).toBe("open");
+      expect(issue.state).toBe("OPEN");
       expect(issue.labels.nodes).toEqual([{ name: "bug" }]);
     });