docs: add launch marketing plan and growth strategy

- marketing/launch-plan.md: 8-week launch timeline, platform strategies
  (Twitter, LinkedIn, HN, Reddit, Dev.to, Product Hunt), content calendar,
  Show HN strategy, thread templates, community seeding, metrics/goals
- marketing/growth-strategy.md: viral loops, SEO/content plan, distribution
  channels, conversion funnel, competitive wedge campaigns, 10 growth
  experiments, realistic solo-dev projections

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: michaelmcnees