feat(cstack): add simple-as-prose

[Camgineer]

Summary

• Add the model-invoked simple-as-prose skill for plain, specific, natural prose.
• Credit Alex Hormozi, @Anthony-Lionetti, and @Camgineer in custom skill metadata.
• Make simple-as-prose the C-Stack writing default for new, resumed, and compacted sessions.
• Update the live skill inventory and bump C-Stack to 0.3.0.

Tests

• npm test
• CSTACK_VERSION_BASE=origin/main npm test
• node --check hooks/scripts/poteto-mode-state.mjs
• Validated the skill frontmatter, author metadata, line count, and absence of long-dash characters.