[github-actions] Resolve consumer action paths in reusable workflows (#180)

[coisa]

Related Issue

• Closes Synchronized workflows reference missing local actions in consumer repositories #180

Motivation / Context

Consumer repositories call the packaged reusable workflows from php-fast-forward/dev-tools, but those workflows were resolving local actions like ./.github/actions/... relative to the caller repository. That made consumer runs fail as soon as a reusable workflow depended on a local action that only exists in this repository.

Changes

• make reusable workflows check out php-fast-forward/dev-tools as a dedicated workflow-action source before using local actions
• resolve local actions from ./.dev-tools-workflow-source/.github/actions/... instead of the caller repository
• keep the source ref stable for consumers while still using the current revision when workflows run inside this repository
• document that consumer repositories do not need mirrored .github/actions directories for these reusable workflows
• add the changelog entry for the workflow-action source checkout behavior

Verification

• ruby -e 'require "yaml"; %w[.github/workflows/auto-assign.yml .github/workflows/label-sync.yml .github/workflows/review.yml .github/workflows/tests.yml .github/workflows/reports.yml .github/workflows/changelog.yml .github/workflows/wiki-preview.yml .github/workflows/wiki-maintenance.yml].each { |path| YAML.load_file(path) }; puts "ok"'
• composer dev-tools changelog:check
• git diff --check

Documentation / Generated Output

• updated docs/usage/github-actions.rst
• updated docs/usage/syncing-consumer-projects.rst

Changelog

• added an Unreleased entry for the reusable workflow action-source checkout fix

Reviewer Notes

• .github/wiki remains local-only and is intentionally not part of this PR

[github-actions]

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