fix: separate label name and color in dispatch guidance (#9056)

[atomantic]

Summary

Fixed malformed repo labels with embedded color hex values (e.g., model:light D4C5F9) that were created by agents copying the color guidance directly as label names.

• Rewrote the color guidance in MANDATORY_DISPATCH_HINT_GUIDANCE to separate label names and hex colors with parentheses and explicit command patterns
• Deleted all 34 malformed labels from the repo (all had 0 usage)
• Updated test to verify colors are included but not combined with names

Test plan

• Verified all malformed labels have been deleted from the repo (0 remaining)
• Tests pass: npm test -- lib/dispatchLabels.test.js (40/40 passing)
• New guidance format explicitly shows label names and colors in separate tokens to prevent miscopying

Verification

• Regex check confirms: gh label list | grep -E ' [0-9A-Fa-f]{6}' returns 0 malformed labels
• Acceptance criteria met: No label name matches / [0-9A-Fa-f]{6}( |$)/
• Guidance text cannot be directly copied into a malformed label name

Closes #9056