[github-actions] Add workflow step summaries (#148)

[coisa]

Related Issue

Closes #148

Motivation / Context

Several of the reusable Fast Forward workflows already publish useful URLs, refs, and verification results, but maintainers still had to reconstruct that information from raw logs or PR comments. This change surfaces the final state in GITHUB_STEP_SUMMARY without replacing the existing logs, comments, or annotations.

Changes

• add a reusable .github/actions/summary/write helper for deterministic GITHUB_STEP_SUMMARY writes
• summarize the resolved PHP version source, matrix, and thresholds in tests.yml
• summarize publish, preview, verification, and cleanup outcomes in reports.yml
• summarize preview, publish, and cleanup outcomes in wiki-preview.yml and wiki-maintenance.yml
• summarize changelog validation, release preparation, and release publication outcomes in changelog.yml
• expose reusable action outputs needed for summary composition in the changelog and cleanup helper actions
• update README, AGENTS, changelog, and GitHub Actions automation docs
• clarify that .github/wiki pointer movement on the branch comes from the wiki preview workflow and is expected workflow-managed state, not unrelated scope drift

Verification

• ruby -e 'require "yaml"; %w[.github/workflows/tests.yml .github/workflows/reports.yml .github/workflows/wiki-preview.yml .github/workflows/wiki-maintenance.yml .github/workflows/changelog.yml .github/actions/summary/write/action.yml .github/actions/changelog/publish-release/action.yml .github/actions/github-pages/cleanup-orphaned-previews/action.yml .github/actions/wiki/cleanup-orphaned-previews/action.yml].each { |path| YAML.load_file(path) }'
• bash -n .github/actions/summary/write/run.sh .github/actions/changelog/publish-release/run.sh .github/actions/github-pages/cleanup-orphaned-previews/run.sh .github/actions/wiki/cleanup-orphaned-previews/run.sh
• composer dev-tools changelog:check
• git diff --check

Documentation / Generated Output

• updated README.md
• updated AGENTS.md
• updated docs/advanced/consumer-automation.rst
• updated docs/usage/github-actions.rst
• updated CHANGELOG.md

Changelog

• Added Add GITHUB_STEP_SUMMARY output to tests, reports, wiki, and changelog workflows for clearer final-state workflow results (#148) to Unreleased

Reviewer Notes

• summary composition stays out of the deploy/publish steps themselves; workflows gather their outputs first and only then call the shared summary helper
• existing PR comments, command logs, and workflow annotations remain intact
• the .github/wiki submodule pointer is updated by the wiki preview workflow itself, so seeing that pointer move on the branch is expected workflow side effect rather than unrelated drift

[github-actions]

🚀 Preview is available for this pull request.

• Docs: https://php-fast-forward.github.io/dev-tools/previews/pr-173/
• Coverage: https://php-fast-forward.github.io/dev-tools/previews/pr-173/coverage/
• Metrics: https://php-fast-forward.github.io/dev-tools/previews/pr-173/metrics/

[coisa]

Additional reviewer context about .github/wiki: this repository updates the wiki submodule pointer as a normal side effect of the wiki preview workflow whenever the PR branch publishes refreshed wiki output. So when that pointer moves on a branch, it is expected workflow-managed state rather than unrelated drift introduced by the feature itself. I wanted to leave that explicit here because the earlier concern treated it as accidental scope creep.