refactor: extract composite actions for eval-lane setup; surviving lanes gain the fail-fast registry verification

'Fix bun temp' x3, 'Restore deps' x5, 'Seed claude interactive config' x3,
and 'Register gstack skills' x3 were byte-near-identical copies across the
legacy matrix, the sliced lane, and the periodic lane — and only the MATRIX
copy of register-skills carried the 19-line dangling-symlink + frontmatter
fail-fast loop written after a silent 'Unknown command' + 35-min-timeout
incident. Extract all four into .github/actions/ composites; the register
composite carries the verification loop (generalized over the skill list),
so the sliced and periodic lanes — the lanes that SURVIVE the matrix
deletion — now inherit the check they had silently dropped. Matrix-job
inline copies are left untouched: that job is deleted next.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/actions/fix-bun-temp/action.yml

@@ -0,0 +1,18 @@
+name: Fix bun temp
+description: >
+  Redirect bun's cache/temp to a runner-writable dir. Bun creates root-owned
+  temp dirs during the Docker image build; GH Actions runs container jobs as
+  the `runner` user with HOME=/github/home, so without this redirect bun hits
+  EACCES on its default temp paths. Shared by every eval lane — extracted from
+  three byte-identical copies (legacy matrix, sliced lane, periodic).
+runs:
+  using: composite
+  steps:
+    - shell: bash
+      run: |
+        mkdir -p /home/runner/.cache/bun
+        {
+          echo "BUN_INSTALL_CACHE_DIR=/home/runner/.cache/bun"
+          echo "BUN_TMPDIR=/home/runner/.cache/bun"
+          echo "TMPDIR=/home/runner/.cache"
+        } >> "$GITHUB_ENV"