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/markdown-links.yml

@@ -1,35 +1,36 @@
 name: Markdown link check
 
 # Verify every internal markdown link in the repo resolves to an existing
-# file (or anchor).  External http(s) links are also probed but with a
-# loose timeout — flaky third-party hosts must not break our CI.
+# file (or anchor).
 #
-# Trigger: PRs that touch any *.md file, plus a weekly cron so external
-# link rot is caught even when nobody is editing docs.
+# Triggers:
+#   - "/links" as a PR comment  (manual, on the PR branch)
+#   - Weekly cron Mon 05:00 UTC (catches link rot without any PR activity)
+#   - workflow_dispatch          (manual run on any branch)
 
 on:
-  pull_request:
-    paths:
-      - "**/*.md"
-      - ".gitea/workflows/markdown-links.yml"
-  push:
-    branches:
-      - main
-    paths:
-      - "**/*.md"
-      - ".gitea/workflows/markdown-links.yml"
+  issue_comment:
+    types: [created]
   schedule:
     - cron: "0 5 * * 1"   # Monday 05:00 UTC weekly link-rot check
   workflow_dispatch: {}
 
 jobs:
   check:
+    if: |
+      github.event_name == 'schedule' ||
+      github.event_name == 'workflow_dispatch' ||
+      (github.event_name == 'issue_comment' &&
+       github.event.issue.pull_request != null &&
+       contains(github.event.comment.body, '/links'))
     runs-on: eulernest
     container:
       image: git.eulernest.eu/conformallab/ci-cpp:latest
 
     steps:
       - uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event_name == 'issue_comment' && format('refs/pull/{0}/head', github.event.issue.number) || github.ref }}
 
       # ── Pure-python internal link check (no external network needed) ────
       # We use the same logic that found the 2 broken links before the