fix: restore release lint gate

[chubes4]

Summary

• bootstrap PHPStan with the external Data Machine and Agents API contracts consumed by the plugin
• fix current PHPCS findings and real static-analysis defects in cleanup, capacity, lifecycle, and process execution paths
• replace the stale repository-wide lint fingerprints with PHPStan's path/message/count baseline for verified legacy findings

Testing

• homeboy --placement local review lint data-machine-code --path /var/lib/datamachine/workspace/data-machine-code@fix-976-release-lint
• 21 focused artifact cleanup, inode capacity, cleanup orchestration, worktree lifecycle, process runner, and mounted-runtime regression scripts
• Homeboy's generic test discovery found no canonical PHPUnit files; this repository uses standalone PHP regression scripts

Closes #976