feat(ci): auto-close problem issues when a solution lands in work/

Reconciles work/ against open `problem` issues instead of diffing the push,
so re-runs are no-ops and backfills need no special casing.

Skips scaffolded stubs: `bun run pick` writes a statement header plus an
empty body, so file existence alone must not close an issue.

.github/workflows/close-solved.yml

@@ -0,0 +1,34 @@
+name: Close Solved
+
+on:
+  push:
+    branches: [main]
+    paths: ["work/**"]
+  workflow_dispatch:
+    inputs:
+      dry_run:
+        description: Report matches without closing anything
+        type: boolean
+        default: false
+
+permissions:
+  contents: read
+  issues: write
+
+concurrency:
+  group: close-solved
+  cancel-in-progress: false
+
+jobs:
+  close:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: oven-sh/setup-bun@v2
+
+      # No install step: the script only uses Bun builtins + fetch.
+      - run: bun scripts/close-solved.ts
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          DRY_RUN: ${{ inputs.dry_run }}