revert(ci): reusable workflows after all — Forgejo 11 supports them

Correction to the previous commit. Forgejo 11.0.15 DOES support reusable
workflows; my earlier "not supported" was a false negative — the test caller
omitted `runs-on`, and the pre-v15 "limited" implementation REQUIRES `runs-on`
on the calling job (omitting it makes Forgejo silently schedule no run). Verified
live: a caller with `runs-on` runs green, same-repo and cross-repo (short ref);
the full-URL form fails for reusable workflows (it was only needed for composite
ACTIONS, which resolve via DEFAULT_ACTIONS_URL).

- Restore the four reusable-*.yml (on: workflow_call), the architecture the
  handover + 999_testing chose; fix the caller examples to include `runs-on`.
- Remove the composite-action layer (actions/) — single mechanism, no redundancy.
- .forgejo/workflows/README.md documents the v11 caller-`runs-on` + short-ref
  quirks (both removed by a future Forgejo v15 upgrade) and the candidate coverage.
- ecosystem-selftest paths filter back to reusable-*.yml.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.forgejo/workflows/reusable-lint.yml

@@ -0,0 +1,64 @@
+# reusable-lint — eslint + yamllint gate (999_testing "linter testing").
+#
+# A REUSABLE workflow (on: workflow_call). Either linter finding an error makes
+# the job exit non-zero (the acceptance criterion). Prefers the project's own
+# pinned eslint (node_modules/.bin) for config/plugin fidelity, falling back to
+# the foundation-ci image's global eslint; yamllint comes from the image.
+#
+#   jobs:
+#     lint:
+#       runs-on: docker   # REQUIRED on Forgejo 11 (pre-v15 reusable-workflow quirk; see README)
+#       uses: olsitec/foundation/.forgejo/workflows/reusable-lint.yml@master
+#       with: { eslint-paths: ".", yamllint-paths: "." }
+name: reusable-lint
+on:
+  workflow_call:
+    inputs:
+      eslint:
+        type: boolean
+        default: true
+      yamllint:
+        type: boolean
+        default: true
+      eslint-paths:
+        type: string
+        default: "."
+      yamllint-paths:
+        type: string
+        default: "."
+      package-manager:
+        description: "bun | npm | none — to install project-local eslint config/plugins"
+        type: string
+        default: bun
+
+jobs:
+  lint:
+    runs-on: docker
+    container:
+      image: foundation-ci:latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install dependencies (project-local eslint config/plugins)
+        if: ${{ inputs.eslint }}
+        run: |
+          case "${{ inputs.package-manager }}" in
+            bun)  bun install --frozen-lockfile || bun install || true ;;
+            npm)  npm ci || npm install || true ;;
+            none) echo "skip install" ;;
+          esac
+
+      - name: eslint
+        if: ${{ inputs.eslint }}
+        run: |
+          if [ -x node_modules/.bin/eslint ]; then
+            echo "+ project eslint"; node_modules/.bin/eslint ${{ inputs.eslint-paths }}
+          else
+            echo "+ image eslint"; eslint ${{ inputs.eslint-paths }}
+          fi
+
+      - name: yamllint
+        if: ${{ inputs.yamllint }}
+        run: |
+          echo "+ yamllint ${{ inputs.yamllint-paths }}"
+          yamllint ${{ inputs.yamllint-paths }}