Add CI workflow, README polish (limitations, development, credits)

[MADEVAL]

Add a proper CI pipeline, expand the README with required GitHub-best-practice sections, and update project metadata.

CI (.github/workflows/ci.yml)

• Lint all PHP files across PHP 7.4, 8.0, 8.1, 8.2, 8.3 and 8.4 on every push and PR.
• Added CI badge to README (3 mandatory badges now: License, Platform, CI).

README sections

• Limitations / Caveats — first-poll load-average approximation, wmic optional-feature note, single-threaded dev-server warning, token exposure disclaimer.
• Development — local server command, lint command, CI workflow reference.
• Credits — Chart.js, Bootstrap.
• Features / Requirements updated to reflect cross-platform (Linux + Windows via WMI).

Repo metadata (applied separately via gh repo edit)

• Description updated to 60–200 chars — longer and more informative.
• Homepage set to https://globus.studio.
• Topics: php, server-monitoring, dashboard, cpu-usage, ram-monitoring, monitoring, chartjs.