[codex] Preserve MCP artifact content on metadata updates

[datashaman]

Summary

Fixes #1.

• Preserve existing artifact content when MCP UpsertArtifact updates only metadata.
• Keep explicit empty strings meaningful so callers can still intentionally clear content.
• Add repo-local agent skill setup docs for issue tracker, triage labels, and domain docs.
• Add MCP regression coverage for views, Livewire SFCs, layouts, snippets, styles, omitted defaults, and explicit blank content.

Root Cause

UpsertArtifact normalized omitted php, blade, and style fields to empty strings before dry-run/write handling. That made metadata-only updates indistinguishable from explicit content-clearing updates.

Validation

• php artisan test tests/Feature/EvolveMcpServerTest.php --filter='metadata_only|distinguishes'
• vendor/bin/pint --dirty --test

Notes

This PR is based directly on main and intentionally does not include the demo blog homepage work from #2.