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/ecosystem-selftest.yml

@@ -13,7 +13,7 @@ on:
     paths:
       - "ci/**"
       - ".forgejo/workflows/ecosystem-selftest.yml"
-      - ".forgejo/workflows/reusable-*.yml"
+      - "actions/**"
   workflow_dispatch:
 
 jobs: