Add solo-skills under Productivity & Organization (#754)

solo-skills is a bilingual (EN + 中文) Claude Code skills pack
shaped for solo founders and indie developers. Includes 7 skills
covering launch tweets, customer emails, changelogs, decision
frameworks, postmortems, and more.

Distinguishing feature: every skill has a mandatory 'When NOT to
use' section, and a fixed output format with worked input/output
examples.