swatches: add a match mode and extra delimiters, per site, language and block

[ewels]

Closes #4

Outside stylesheets, a colour gets a swatch only when it looks like a value: after :, =, ,, ( or a quote, and before ,, ;, ), }, ] or the end of the line. That misses colours in syntaxes such as nf-metro's %%metro line: main | Main | #4CAF50, and a site had no way to fix it.

New swatches options:

• match: 'value' | 'all' (default 'value'). 'all' gives every hex colour and colour function in a block a swatch, as in stylesheets. The guards stay on: #123 / #1234 and short all-letter hex such as #add still get no swatch outside quotes, and named colours still need quotes.
• delimiters: { before?: string[]; after?: string[] } (default {}). Extra text allowed before and after a colour that looks like a value. It adds to the built-in delimiters.
• byLanguage: Record<string, { match?, delimiters? }> (default {}). Per-language settings, which replace the site match and delimiters for that language. languages stays as it is (it picks which blocks are scanned), because a map there could not say "all languages, plus these settings".
• Fence attribute swatches.match="all" / swatches.match="value" for one block. A bad value warns and the block uses the site or language setting.

Defaults are unchanged. The decision and the rejected shapes are in .agents/DECISIONS.md. The docs page, the options and attributes reference and the skill are updated.

pnpm lint, pnpm test, pnpm lint:docs and pnpm docs:build pass with no warnings.

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