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-node-build.yml

@@ -1,58 +0,0 @@
-# reusable-node-build — build/test an npm- or bun-based project (999_testing).
-#
-# A REUSABLE workflow (on: workflow_call) downstream repos call:
-#   jobs:
-#     build:
-#       uses: olsitec/foundation/.forgejo/workflows/reusable-node-build.yml@master
-#       with: { package-manager: bun, build: "bun run build" }
-#
-# Runs in the baked foundation-ci image (bun + node present). Covers the
-# non-Docker candidate shapes: npm package built with npm (olsicrypto), bun
-# package built with bun (document-engine), and the no-build / versioned-only
-# utility (olsitrack/api) via an empty `build`.
-name: reusable-node-build
-on:
-  workflow_call:
-    inputs:
-      package-manager:
-        description: "bun | npm | none (none = skip install)"
-        type: string
-        default: bun
-      build:
-        description: "build command to run verbatim (empty = skip, e.g. no-artifact repos)"
-        type: string
-        default: ""
-      workdir:
-        description: "working directory for install + build"
-        type: string
-        default: "."
-
-jobs:
-  build:
-    runs-on: docker
-    container:
-      image: foundation-ci:latest
-    defaults:
-      run:
-        working-directory: ${{ inputs.workdir }}
-    steps:
-      - uses: actions/checkout@v4
-
-      - name: Install dependencies (${{ inputs.package-manager }})
-        run: |
-          case "${{ inputs.package-manager }}" in
-            bun)  bun install --frozen-lockfile || bun install ;;
-            npm)  npm ci || npm install ;;
-            none) echo "package-manager=none → skipping install" ;;
-            *)    echo "unknown package-manager '${{ inputs.package-manager }}'" >&2; exit 1 ;;
-          esac
-
-      - name: Build
-        run: |
-          cmd='${{ inputs.build }}'
-          if [ -z "$cmd" ]; then
-            echo "no build command (non-artifact / versioned-only repo) — install-only check passed"
-            exit 0
-          fi
-          echo "+ $cmd"
-          eval "$cmd"