refactor: split guardrails extensions

[aliou]

Summary

Refactors guardrails into a split extension architecture:

• src/core: Pi-free primitives and typed rule checks.
• src/shared: shared config, migrations, events, matching, warnings, and path helpers.
• extensions/guardrails: policy enforcement plus settings, onboarding, and examples commands.
• extensions/path-access: outside-workspace path access control.
• extensions/permission-gate: dangerous command confirmation.

Also updates the package to Pi 0.74.0 / @earendil-works/*, restores and reorganizes the TUI commands, adds focused tests for the split features, updates docs, and adds a local skill for building future guardrails extensions.

User-facing changes

• Adds /guardrails:examples for applying policy and dangerous-command presets.
• Keeps /guardrails:settings as the main settings UI.
• Registers /guardrails:onboarding only when onboarding is still pending.
• Keeps all split extensions sharing guardrails.json.
• Updates docs for defaults, examples, config, and Pi 0.74 package scope migration.

Validation

• pnpm typecheck
• pnpm lint
• pnpm test -- --runInBand
• Manual TUI smoke tested successfully:
  • /guardrails:settings
  • /guardrails:examples
  • onboarding command visibility after completion

Still left before merge

• Add changeset.
• Optional later hardening: user regex ReDoS mitigation.
• Optional later cleanup: harmonize guardrails event payloads with the new registry-style event shape.

Leaving this PR as draft until release prep is done.