fix(agent): make CSS edits replaceable and priority-aware

[DavidBabinec]

Summary

• add explicit merge, replace, delete, and remove-properties operations to site_apply_css with exact-selector, atomic store semantics
• preserve !important through CSS import, persistence, canvas preview, responsive sizing, and published output
• expose background image/clip and WebKit text-mask computed styles in render snapshots so cascade resets are diagnosable
• advertise a flat provider-compatible CSS tool schema while enforcing the exact operation union at the editor boundary

Why

A later background shorthand could reset an earlier background-clip: text, leaving transparent text over a solid gradient rectangle. The previous merge-only tool could not reliably remove or replace stale declarations, and CSS priority metadata was discarded.

Anthropic also requires tool inputs to be object-rooted with no root-level anyOf, oneOf, or allOf. The provider schema now follows that contract without weakening operation-specific validation in the editor.

User/developer impact

Agents can repair overlapping CSS without accumulating more rules, destructive edits are explicit and atomic, class identity and assignments survive replacement, and snapshots expose the computed evidence needed to debug gradient text. The same tool now works across Anthropic and the other supported providers.

Verification

• bun test — 6,206 pass, 0 fail
• focused post-rebase agent integration suite — 176 pass, 0 fail
• bun run lint
• bunx tsc --noEmit
• bun run build
• GitHub CI + CodeQL — 7 checks pass