Add minimal /caveman command

[romgenie]

Summary

• add a focused /caveman command helper for mode parsing, prompt construction, and advertised client-tool suppression
• wire /caveman [mode] in the interactive CLI as a conversation-local mode switch
• register /caveman in built-in command handling with an explicit non-interactive not-applied result
• add focused command tests

Intentional minimal scope

This keeps the first implementation narrow. It does not add the caveman personality preset, bundled skill, prompt asset, agents --personality cave-code, broad approval/interrupt continuation hardening, or unrelated cleanup from the larger PR line.

Validation

• bun test src\tests\cli\caveman-command.test.ts
• bun run typecheck
• bunx --bun @biomejs/biome@2.2.5 check src/cli/App.tsx src/cli/commands/registry.ts src/cli/commands/caveman.ts src/tests/cli/caveman-command.test.ts
• git diff --check HEAD~1..HEAD

Known base-state checks left out of scope:

• bun run lint reports existing warnings in src/agent/reconcileExistingAgentState.ts, src/tests/channels/runtimeDeps.test.ts, and src/web/generate-memory-viewer.ts.
• bun run build fails on Windows because current main still shells out to chmod in build.js.

Closes #20.