feat(caveman): minimal cave-code personality + /caveman command

[romgenie]

Summary

Minimal additive variant of #19. Ships the cave-code personality preset, /caveman slash command, and bundled skill without the clientTools suppression plumbing and without the incidental cleanups.

Closes #21.

Scope

• New src/agent/prompts/persona_caveman.mdx and src/skills/builtin/caveman/SKILL.md
• Register persona_caveman.mdx in promptAssets.ts and document in prompts/README.md
• Add caveman to PersonalityId, PERSONALITY_OPTIONS, PERSONALITY_ALIASES (cave-code), and the existing branch-based lookups in personality.ts (no table-lookup refactor)
• New src/cli/commands/caveman.ts with CAVEMAN_MODES, normalizeCavemanMode, buildCavemanCommandPrompt, isCavemanCommandInput. Exports no suppressPreparedClientTools helper.
• /caveman dispatch in src/cli/App.tsx that calls processConversation with the mode-switch prompt — no new refs, no reentryOptions, no queued-approval metadata changes, no cancel-path reset
• Register /caveman in commands/registry.ts with a plain-string handler (no CommandResult type widening)
• Update --personality help text in subcommands/agents.ts to include cave-code
• personality.test.ts additions + focused caveman-command.test.ts (mode parsing, prompt building, SKILL.md example alignment, slash-command registration)

Deliberately out of scope

Left for follow-up PRs so each can be reviewed on its own merits:

• Hard clientTools: [] suppression on the mode-switch turn (the persona + skill + prompt already tell the agent not to call tools; only add enforcement if the model ignores the instruction in practice)
• personality.ts refactor from ternary chains to PERSONA_TEMPLATE_BY_ID / HUMAN_TEMPLATE_BY_ID / PERSONA_CONTENT_OVERRIDES tables
• registry.ts handler return-type widening to string | CommandResult
• Incidental cleanups bundled into Add cave-code command and skill support #19: build.js Windows chmodSync, reconcileExistingAgentState.ts dead helper, generate-memory-viewer.ts ConversationListItem typing, runtimeDeps.test.ts dead helper

Size

11 files changed, +480/-7. Net feature is 4 new files (persona, skill, command module, test file) plus small additions in 7 existing files.

Validation

• bun run typecheck — clean
• bun run lint — 3 pre-existing warnings on main, none from this change
• bun run build — clean
• bun test src/tests/agent/personality.test.ts src/tests/cli/caveman-command.test.ts — 19/19 pass

Test plan

• bun run typecheck is clean
• bun run lint shows no new warnings
• bun run build produces a letta.js that loads
• bun test src/tests/agent/personality.test.ts src/tests/cli/caveman-command.test.ts all pass
• Interactive smoke test: /caveman, /caveman ultra, /caveman wenyan-full all switch modes and the agent responds in the selected style; no tool calls during the switch turn
• letta agents create --personality cave-code creates an agent with the caveman persona

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