ci: migrate all workflows to Ubicloud (Linux + Windows, 8-core)

Switch every `runs-on` in this repo to Ubicloud so CI has a single billing
surface, consistent capacity, and 4x more cores on the workloads that were
previously stuck on free `ubuntu-latest` (2 cores). Windows uses Ubicloud's
Windows pool too — `ubicloud-standard-8-windows` — so the queued-forever
problem with GitHub's `windows-latest-8-cores` paid larger runner (org-level
larger-runner billing not enabled) goes away.

Workflows touched (9):
- evals.yml, evals-periodic.yml, ci-image.yml — bump default + matrix from
  `ubicloud-standard-2` to `ubicloud-standard-8`. The one matrix entry that
  was already on -8 stays.
- windows-free-tests.yml — `ubicloud-standard-2-windows` → `ubicloud-standard-8-windows`.
- make-pdf-gate.yml — matrix `ubuntu-latest` → `ubicloud-standard-8`. macOS
  entry preserved; the poppler-install `if: matrix.os` conditional swaps to
  match the new label.
- actionlint.yml, pr-title-sync.yml, skill-docs.yml, version-gate.yml —
  `ubuntu-latest` → `ubicloud-standard-8`.

.github/actionlint.yaml registers all four Ubicloud labels in one place:
- ubicloud-standard-2
- ubicloud-standard-8
- ubicloud-standard-2-windows  (the v1.38.0.0 windows-free-tests target)
- ubicloud-standard-8-windows  (this PR's windows-free-tests target)

Removed the duplicate `actionlint.yaml` at the repo root that I accidentally
created in the prior commit — actionlint only reads `.github/actionlint.yaml`,
so the root file was dead weight.

CHANGELOG entry updated: a single "all Ubicloud" sentence in the narrative
plus a metrics-row covering the runner pool change, and the itemized line
expanded to enumerate the 9 affected workflows. The previously-orphaned
"Itemized changes" line about just `windows-free-tests.yml` is replaced.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.github/workflows/windows-free-tests.yml

@@ -8,10 +8,10 @@ name: Windows Free Tests
 # targeted resolver tests that exercise the Bun.which-based claude binary
 # resolution + the GSTACK_CLAUDE_BIN override path on Windows.
 #
-# Runner: Ubicloud Windows runner (`ubicloud-standard-2-windows`). Matches
-# the Ubicloud Linux runner the evals workflow already uses, so billing
-# stays consolidated under one provider. Swap to `windows-latest` to revert
-# to GitHub's free runner if Ubicloud has Windows capacity issues. Note
+# Runner: Ubicloud Windows runner (`ubicloud-standard-8-windows`). All CI
+# in this repo runs on Ubicloud (Linux + Windows) for billing consolidation
+# and consistent capacity. Swap to `windows-latest` to revert to GitHub's
+# free runner if Ubicloud has Windows capacity issues. Note
 # `windows-latest-8-cores` (GitHub paid larger runner) sat queued because
 # org-level larger-runner billing wasn't enabled.
 #
@@ -36,7 +36,7 @@ jobs:
   windows-free-tests:
     # Ubicloud Windows runner (same provider as the Linux evals workflow).
     # To revert: swap to `windows-latest` (GitHub's free 4-core Windows runner).
-    runs-on: ubicloud-standard-2-windows
+    runs-on: ubicloud-standard-8-windows
     timeout-minutes: 15
 
     steps: