feat(theme): write and validate a theme's syntax and pair

[omartelo]

The theme skill learns the two optional keys the native window reads.

• syntax: the skill ports a palette's code colors from its editor theme (a JetBrains color scheme or a VS Code tokenColors) into highlighting tags, and names the tags lich gives Java's enum constants, annotations, fields and import paths.
• pair: the skill pairs a palette's light and dark variants so lich native follows the desktop between them with Auto light/dark on.

validate.mjs previously ignored syntax. It now checks both keys as lich's validateSyntax and checkPairs do:

• an unknown highlighting tag or modifier;
• a style with no color and no font style;
• a color outside the app color rule;
• a theme that is its own pair;
• a pair of the same scheme, whether a sibling in the pack or bundled light/dark.

tests/lich-ref moves from v0.65.0 to omartelo/lich@c42e0ea2, the commit whose template shows pair beside syntax. No lich release ships them yet, which is the unreleased-contract exception in CLAUDE.md. That commit is on feat/native-ui, not a merge on main. The hook fixtures are byte-identical at that ref; only skills/theme/template.json moved. When a lich release ships syntax and pair, the ref goes back to its tag.

Tests: node --test tests/*.test.mjs, 427 pass, 8 of them new for the validator. The published packs (everforest, catppuccin, rose-pine, eldritch, lotus, bluloco) validate clean.