refactor(ci): composite actions instead of reusable workflows (Forgejo 11)

Forgejo 11.0.15 does NOT support reusable workflows (job-level `uses:` /
`workflow_call`): the call is silently dropped and no run is scheduled (verified
live — a same-repo and a cross-repo reusable call both produced zero runs, while
an equivalent inline job ran green). The working cross-repo reuse primitive here
is the COMPOSITE ACTION referenced by FULL URL (a short-form
`uses: olsitec/foundation/...@master` resolves against the runner's
DEFAULT_ACTIONS_URL = data.forgejo.org, not the local instance, and 404s; the
full-URL form `uses: https://forge.olsitec.net/olsitec/foundation/actions/<x>@master`
was verified green).

- Replace the four reusable-*.yml with composite actions under actions/:
  node-build, docker-build, lint, semantic-release-version (same logic + inputs).
- actions/README.md documents the pattern, the Forgejo-11 limitation, and the
  999_testing candidate coverage (C2/C3/C4 self-contained; C1/C5 blocked on the
  not-yet-published @olsitec package registry).
- ecosystem-selftest paths filter: actions/** (was reusable-*.yml).

The capabilities that need no external repo (semantic-release bump sequence,
eslint/yamllint gates) keep running green via ecosystem-selftest's inline jobs.

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

.forgejo/workflows/reusable-lint.yml

@@ -1,63 +0,0 @@
-# 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:
-#       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 }}