feat: color lines by special tag

[vycdev2]

Summary

• add opt-in whole-line background colors for special tags through todo.colors.tags.lineBackground
• select the first colored tag on each line deterministically while preserving existing status text colors and tag styling
• repeat shorter base, dark, and light line-background palettes across all configured tags
• document the setting and cover its schema, range selection, precedence, theme overrides, and short-palette fallback

Verification

• npm test — passed: 231 tests
• npm run compile — passed: webpack development build completed
• npm run format:check — passed: all matched files use Prettier formatting
• git diff --check upstream/develop...HEAD — passed

Risk

• Low: the new palette defaults to empty, so existing decoration behavior is unchanged until configured.

Closes #93

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.

[vycdev]

The new special-tag palette needs to follow the palette fallback semantics already on develop.

[vycdev]

Current develop makes shorter special-tag palettes repeat across every configured tag. Indexing lineBackground directly means ['red'] colors only the first tag while the existing background/foreground palettes color all tags, and the dark/light arrays have the same issue below. Please use the shared palette helper for base, dark, and light line backgrounds and add a short-palette regression.

[vycdev2]

Addressed the short-palette feedback and rebased onto current develop. Whole-line base, dark, and light palettes now use the shared repeating palette helper, with a regression covering a third configured tag reusing a two-color palette.

Verification: npm test (231 passing), npm run compile, npm run format:check, and git diff --check upstream/develop...HEAD all pass. Updated commit: 3e3583d.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.