ci: switch all non-fast jobs to comment triggers

Every CI job except test-fast and mirror-to-codeberg now runs only
when explicitly requested via a PR comment, instead of on every push
or PR sync.  This keeps the Pi runner idle during WIP commits and lets
the author decide when to pay each job's cost.

Trigger commands:
  /test-cgal       → CGAL test suite (277 tests, ~5 min build + 31 s run)
  /quality-gates   → license/codespell/shellcheck/cgal-conventions (~30 s)
  /docs            → Doxygen build + warning summary (~2 min)
  /links           → Markdown internal link check (~10 s)

All comment-triggered jobs check:
  event is issue_comment
  AND comment is on a PR (issue.pull_request != null)
  AND comment body contains the trigger word
  AND checkout uses refs/pull/N/head (not the default branch)

Jobs that stay automatic:
  test-fast         — runs on every push (26 pure-math tests, < 5 s)
  mirror-to-codeberg — unchanged

Jobs that keep additional triggers:
  markdown-links — weekly cron (Mon 05:00 UTC) + workflow_dispatch
  doc-build      — workflow_dispatch (for manual runs outside a PR)

quality-gates drops `needs: test-fast` — it now runs independently
when comment-triggered (caller decides whether test-fast passed first).

CLAUDE.md CI pipeline table updated with all five jobs and their new
trigger descriptions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/cpp-tests.yml

@@ -152,25 +152,25 @@ jobs:
 # ─────────────────────────────────────────────────────────────────────────────
 # Job 3 — quality-gates  (style + convention block)
 #
-# Cheap, deterministic checks that should never break unless a contributor
-# introduces a regression.  Each gate is a script under scripts/quality/
-# and exits 0 only when its tree is clean.  These ran for weeks locally
-# at zero findings before being promoted here.
+# Trigger: write "/quality-gates" as a comment on any pull request.
 #
-# Tools installed at job-start (the ci-cpp image already has python3 +
-# bash; we add codespell + shellcheck on top).  Total wall-time: ~30 s
-# on the eulernest runner.
-#
-# Strictly required for merges into main/dev — a regression fails the PR.
+# Cheap, deterministic checks (~30 s): license headers, CGAL conventions,
+# codespell, shellcheck.  Runs independently of test-fast when comment-
+# triggered (no `needs:` — the caller decides when to invoke it).
 # ─────────────────────────────────────────────────────────────────────────────
   quality-gates:
-    needs: test-fast
+    if: |
+      github.event_name == 'issue_comment' &&
+      github.event.issue.pull_request != null &&
+      contains(github.event.comment.body, '/quality-gates')
     runs-on: eulernest
     container:
       image: git.eulernest.eu/conformallab/ci-cpp:latest
 
     steps:
       - uses: actions/checkout@v4
+        with:
+          ref: refs/pull/${{ github.event.issue.number }}/head
 
       - name: Install codespell + shellcheck (job-local)
         run: |