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/evals.yml

@@ -15,7 +15,7 @@ env:
 jobs:
   # Build Docker image with pre-baked toolchain (cached — only rebuilds on Dockerfile/lockfile change)
   build-image:
-    runs-on: ubicloud-standard-2
+    runs-on: ubicloud-standard-8
     permissions:
       contents: read
       packages: write
@@ -56,7 +56,7 @@ jobs:
             ${{ env.IMAGE }}:latest
 
   evals:
-    runs-on: ${{ matrix.suite.runner || 'ubicloud-standard-2' }}
+    runs-on: ${{ matrix.suite.runner || 'ubicloud-standard-8' }}
     needs: build-image
     container:
       image: ${{ needs.build-image.outputs.image-tag }}
@@ -155,7 +155,7 @@ jobs:
           retention-days: 90
 
   report:
-    runs-on: ubicloud-standard-2
+    runs-on: ubicloud-standard-8
     needs: evals
     if: always() && github.event_name == 'pull_request'
     timeout-minutes: 5
@@ -219,7 +219,7 @@ jobs:
           $(echo -e "$SUITE_LINES")
 
           ---
-          *12x ubicloud-standard-2 (Docker: pre-baked toolchain + deps) | wall clock ≈ slowest suite*"
+          *12x ubicloud-standard-8 (Docker: pre-baked toolchain + deps) | wall clock ≈ slowest suite*"
 
           if [ "$FAILED" -gt 0 ]; then
             FAILURES=""