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/restore-deps/action.yml

@@ -0,0 +1,19 @@
+name: Restore deps
+description: >
+  Restore the CI image's pre-installed node_modules via recursive copy, or
+  fall back to bun install when the lockfile changed. Symlinking breaks bun's
+  realpath-based module resolution (realpath escapes the workspace and
+  sibling deps stop resolving); hardlink copy fails across overlay-fs layers
+  ("Invalid cross-device link"). Recursive copy costs ~5s for ~200 packages —
+  still far cheaper than a network install. Extracted from five byte-similar
+  copies across the eval lanes.
+runs:
+  using: composite
+  steps:
+    - shell: bash
+      run: |
+        if [ -d /opt/node_modules_cache ] && diff -q /opt/node_modules_cache/.bun.lock bun.lock >/dev/null 2>&1; then
+          cp -r /opt/node_modules_cache node_modules
+        else
+          bun install
+        fi