Add Color Swatches plugin

[brsbl]

What changed

This adds Color Swatches, a frontend content script that decorates color literals in thread timelines with IDE-style chips. It recognizes valid hex colors and modern CSS color functions, validates matches through CSS.supports, shows alpha over a checkerboard, and excludes editable composer surfaces.

The implementation does not split or wrap React-owned text nodes. It annotates an existing token boundary and draws the chip with ::before, preserving streaming updates, selection, and copied text.

The plugin builds against the released @get-bb/plugin-sdk 0.4.8 package while retaining engines.bbPluginSdk: ^0.4.1 and engines.bb: >=0.0.34.

Verification

• npm run check --workspace=bb-plugin-color-swatches
• node tooling/check-repository.mjs
• Fixture content script reports 0 swatches on the parent and 13 on this branch
• Mobile, narrow, normal, and ultrawide captures with the same first color block held at the same viewport position

Evidence

Before — parent branch without timeline swatches

After — this branch with IDE-style swatches

Full visual evidence

BB-Thread-ID: thr_tkwwfu5nas