Add the stage facade and run test-staging through it

[alexadereyko]

Depends on:

• Add staging-push workflow to push stagings to ghcr #7
• Add staging-pull and pull the core through ghcr in the test #8

Brief

Add stage, a reusable workflow that composes the three primitives — staging-pull, reusable, staging-push — into one publish flow, so a project builds and publishes its staging in a single call.

Description

stage runs three steps: pull dependencies from ghcr (optional) → build / stage / test (reusable) → publish to ghcr (optional). Each side is gated by an empty input: no staging-pull means no pull, no staging-push means no push (the build's staging just stays a run artifact).

The inputs are a pass-through to the primitives:

• staging-pull — the deps array for staging-pull (empty → no pull).
• include-jobs, cmake-presets, cpack-presets, packages, upload-pattern, timeout — passed straight to reusable.
• staging-push — the stagings array for staging-push (empty → no push).

Usage example

A module that depends on core is two stage calls — core builds and publishes itself, then the module pulls it back from ghcr and builds against it:

jobs:
  core:
    permissions: { contents: read, packages: write }
    uses: ./.github/workflows/stage.yml
    with:
      include-jobs:   '[ ... ]'
      cmake-presets:  '[ ... ]'
      cpack-presets:  '[ ... ]'
      staging-push:   '[ { "path": "ghcr.io/<org>/.../core", "artifacts": ["core-staging-*"] } ]'

  module:
    needs: core
    permissions: { contents: read, packages: write }
    uses: ./.github/workflows/stage.yml
    with:
      staging-pull:   '[ { "artifact": "dep-core-<job>", "packages": [ ... ] } ]'
      cmake-presets:  '[ ... ]'
      packages:       '[ { "artifacts": [ { "name": "dep-core-<job>", "dir": "staging-deps" } ] } ]'
      staging-push:   '[ { "path": "ghcr.io/<org>/.../module", "artifacts": ["module-staging-*"] } ]'