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/seed-claude-config/action.yml

@@ -0,0 +1,30 @@
+name: Seed claude interactive config
+description: >
+  Seed ~/.claude.json with onboarding-complete plus the API-key approval so
+  PTY sessions never wedge on the fresh-container onboarding / "use detected
+  ANTHROPIC_API_KEY?" dialog. Mirrors what the hermetic E2E child env seeds.
+  Idempotent — safe to run unconditionally in sliced lanes where any slice
+  can host a PTY test. Only the key's last 20 chars are persisted (the
+  approval-hash form claude itself writes), never the whole key.
+inputs:
+  anthropic-api-key:
+    description: API key whose suffix gets pre-approved. Pass from secrets at the call site.
+    required: false
+    default: ''
+runs:
+  using: composite
+  steps:
+    - shell: bash
+      env:
+        ANTHROPIC_API_KEY: ${{ inputs.anthropic-api-key }}
+      run: |
+        node -e '
+          const fs = require("fs"), os = require("os"), path = require("path");
+          const p = path.join(os.homedir(), ".claude.json");
+          const seed = fs.existsSync(p) ? JSON.parse(fs.readFileSync(p, "utf8")) : {};
+          seed.hasCompletedOnboarding = true;
+          const key = process.env.ANTHROPIC_API_KEY || "";
+          if (key) seed.customApiKeyResponses = { approved: [key.slice(-20)], rejected: [] };
+          fs.writeFileSync(p, JSON.stringify(seed, null, 2));
+          console.log("seeded", p);
+        '