ci: remove /ci-all — Pi cannot sustain parallel Docker containers

The Pi runner (3-4 GB RAM, swap heavily loaded) OOMs when /ci-all
triggers test-cgal + quality-gates + doc-build + links simultaneously:
four Docker containers start at once after test-fast completes, each
consuming 150-400 MB, exhausting available RAM.

Fix: /ci-all removed from all three workflow files.  Each keyword
now triggers exactly one job — no parallel container competition.

Safe workflow: one keyword per commit.
  Typical sequence:
    git commit -m 'fix: ... /test-cgal'   → CGAL tests (~5 min)
    git commit -m 'chore: /quality-gates' → style checks (~30 s)

CLAUDE.md: CI table updated with Pi-runner warning note.

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

.gitea/workflows/doc-build.yaml

@@ -27,8 +27,7 @@ jobs:
   doc-build:
     if: |
       github.event_name == 'workflow_dispatch' ||
-      contains(github.event.head_commit.message, '/docs') ||
-      contains(github.event.head_commit.message, '/ci-all')
+      contains(github.event.head_commit.message, '/docs')
     runs-on: eulernest
     container:
       image: git.eulernest.eu/conformallab/ci-cpp:latest