[sync] Package agents payloads and relative links (#188)

[coisa]

Related Issue

Closes #188

Motivation / Context

• consumer repositories could not find packaged .agents/agents content after installing fast-forward/dev-tools
• synchronized skill and agent links were being written as absolute machine-local paths, which made generated metadata invalid for other contributors

Changes

• treat extra.gitattributes.keep-in-export parent paths as preserving nested export-ignore candidates too, so keeping /.agents/ also protects packaged /.agents/agents/ content
• normalize the repository .gitattributes to stop exporting away the packaged agent payload
• create repository-relative symlink targets in the packaged directory synchronizer instead of absolute filesystem paths
• fix .gitattributes writer normalization so gitattributes --check no longer reports false drift from an extra trailing blank line
• update the skills and agents command docs to describe repository-relative symlink behavior

Verification

• composer dev-tools
• Focused command(s):
  • ./vendor/bin/phpunit tests/GitAttributes/ExportIgnoreFilterTest.php tests/GitAttributes/MergerTest.php tests/GitAttributes/WriterTest.php tests/Sync/PackagedDirectorySynchronizerTest.php tests/Console/Command/AgentsCommandTest.php tests/Console/Command/SkillsCommandTest.php
  • composer dev-tools gitattributes -- --check
  • composer dev-tools changelog:check
  • git diff --check
• Manual verification:

Documentation / Generated Output

• README updated
• docs/ updated
• Generated or synchronized output reviewed

Changelog

• Added a notable CHANGELOG.md entry

Reviewer Notes

• The package-export fix is split between the keep-in-export matching logic and the normalized .gitattributes output, so this PR intentionally touches both the generator and the generated file.

[github-actions]

🚀 Preview is available for this pull request.

• Docs: https://php-fast-forward.github.io/dev-tools/previews/pr-194/
• Coverage: https://php-fast-forward.github.io/dev-tools/previews/pr-194/coverage/
• Metrics: https://php-fast-forward.github.io/dev-tools/previews/pr-194/metrics/