feat(linter): focus-ring tokens and on-* color pairing

[drmikecrypto]

Summary

• Add focusRingColor, focusRingWidth, and focusRingOffset component property tokens with spec guidance (preserve browser default when omitted; WCAG 2.4.11)
• Add focus-ring-contrast lint rule (3:1 against backgroundColor; 2px width floor)
• Add on-color-contrast and missing-on-color rules for background/on-* token pairs
• Extend recommended color token names with on-primary, on-secondary, on-error

Test plan

• bun test focus-ring-contrast, missing-on-color, on-color-contrast, types, spec-config

Addresses #116 (items 4 and 5)

[securecat]

Thank you for picking this up so quickly — items 4 and 5 are addressed cleanly here, and the "preserve browser default when tokens are absent" framing in item 5 is exactly the guardrail I was hoping for.

One housekeeping note: the commit message currently contains Fixes #116, which will cause GitHub to automatically close issue #116 when this PR is merged. However, items 1–3 and 6–10 from that issue are not addressed in this PR. It may be worth changing the reference to Addresses #116 (items 4 and 5) to keep the issue open for the remaining work.

Thanks again — looking forward to seeing this land.

[drmikecrypto]

Good catch — updated the commit message and PR description to Addresses #116 (items 4 and 5) so the issue stays open for the remaining items. Applied the same Addresses wording on #141 and #142 for their respective item ranges as well.