Add cave_code caveman personality preset

[romgenie]

Summary

Add cave_code, a caveman-style personality preset for Letta Code agents.

Desired behavior

• Users can create an agent with the caveman personality preset.
• The preset presents itself as cave_code.
• cave_code keeps answers terse, exact, and caveman-styled while preserving technical accuracy.
• The preset includes the provided caveman persona, bundled skill, and command support.

Task list

• Add caveman personality preset that presents as cave_code.
• Register persona_caveman.mdx through MEMORY_PROMPTS.
• Add bundled caveman skill with parseable name and description frontmatter.
• Make /caveman a built-in slash command that ships with installed Letta Code, not only a project-local .commands file.
• Add tests for /caveman command prompt generation and command handling.
• Update PR summary/validation after command implementation.
• Interface-test the /caveman modes against the live cave_code agent.

Acceptance criteria

• --personality caveman resolves successfully.
• cave_code uses persona_caveman.mdx for the persona block.
• Caveman aliases resolve through the personality helper.
• Existing default curated create-agent personalities remain unchanged.
• /caveman [mode] is available as a shipped slash command.
• Focused personality and command tests pass.
• Live interface mode switching works for lite, full, ultra, and wenyan variants.