Add minimal /caveman interactive mode switch

[romgenie]

Goal

Add the /caveman interactive CLI command with the smallest practical change set, instead of bundling persona presets, skills, broader approval lifecycle hardening, and unrelated cleanup in the same PR.

Minimal scope

• Add a small caveman command helper for mode parsing and prompt construction.
• Wire /caveman [mode] in the interactive CLI to send a conversation-local mode-switch reminder.
• Suppress advertised client tools for the mode-switch request itself.
• Register /caveman so built-in command handling/help can recognize it.
• Add focused tests for mode parsing, prompt construction, registry behavior, and tool-advertisement suppression.

Explicitly out of scope for the minimal PR

• New caveman personality preset.
• persona_caveman.mdx prompt asset.
• Bundled src/skills/builtin/caveman/SKILL.md.
• agents --personality cave-code support.
• Broad approval/interrupt continuation hardening beyond the mode-switch request itself.
• Unrelated cleanup such as build-script fixes, dead helper removal, or memory-viewer typing cleanup.

Notes

This intentionally keeps /caveman as a conversation-local interactive mode switch. Follow-up PRs can add the persona preset, bundled skill, and deeper approval/interrupt behavior if needed.