Expand build-in-public guidance inside marketing-plan

[howardpen9]

Summary

Follows up on feedback from closed #22 ("Better to update the marketing skill itself. Don't want more skills than chapters.").

• Folds build-in-public guidance into skills/marketing-plan/SKILL.md instead of adding an 11th skill
• No README / skill-count / plugin.json changes — still 10 skills
• Adds: what to share / not share, transparency audit, failure-in-public protocol, anti-patterns, and a sharper build-in-public output block

What changed

Only skills/marketing-plan/SKILL.md:

• Description now triggers on radical transparency / build-in-public
• New Build in Public section under the existing social-media guidance
• Output item 4 expanded into a concrete plan (today's honest post, monthly cadence, failure protocol)

Test plan

• Open skills/marketing-plan/SKILL.md and confirm tone matches other skills (short, actionable)
• Confirm repo still has 10 skill folders (no skills/build-in-public/)
• Confirm README still says "all 10 skills"
• Trigger skill with something like "I want to build in public for my SaaS" and verify output covers the new section

Closes the gap called out in #22 without growing the skill set past the book chapters.

[test-confidence]

⏳ Test Confidence: 0.0%

░░░░░░░░░░░░░░░░░░░░ 0.0%
0 tests passed

Waves
⏳ 80% — 0 tests — Documentation-only change to a marketing skill markdown file. No code paths affected, no tests exist for this file.
⬜ 95% — 0 tests — No related code or integration tests to run for a pure markdown content update.
⬜ 99% — 0 tests — No feature specs exist that exercise skill markdown content; change has no runtime effect.
⬜ 100% — ~0 remaining — Full suite for mathematical certainty that no test inadvertently references this skill's content.

RUNNING — Wave 1 of 4