[command] Add dependency analysis command (#10)

[coisa]

Summary

Adds a new dependencies command that combines composer-dependency-analyser and composer-unused into a single dependency health report. Both analyzers are now bundled as direct dependencies of fast-forward/dev-tools, so the command works out of the box after installing this package.

Changes

• add FastForward\DevTools\Command\DependenciesCommand and register it in the Composer command provider
• add focused PHPUnit coverage for success, findings, unreadable analyzer output, and missing-binary installation failures
• bundle shipmonk/composer-dependency-analyser and icanhazstring/composer-unused in composer.json and update user-facing docs accordingly

Testing

• composer dev-tools tests -- --filter='DependenciesCommandTest|DevToolsCommandProviderTest'
• composer dev-tools

Closes #10