feat: celebrate karma milestones

[variablenix]

Summary

• confirm and document persistent Daily consecutive-day streaks
• celebrate positive Karma threshold crossings at +10, +25, +50, and +100
• highlight milestone notices with a trophy and IRC color
• preserve existing positive, negative, and mixed update decorations
• add regression coverage for milestone crossing behavior

Verification

• go test ./...
• go test -race ./...
• go vet ./...
• go mod verify
• git diff --check