feat(ls): allow host-controlled guest commands

[kermanx]

Summary

• add the machine-scoped p2p-live-share.languages.allowGuestCommandControl host setting, disabled by default
• advertise the internal LSP command bridge so guest language actions can call back into the host
• forward completion, code-action, bare command, and code-lens commands through opaque per-peer capabilities
• enforce the setting and capability lookup on the host before execution

Verification

• pnpm typecheck
• pnpm exec eslint src/configs.ts src/ls/guest.ts src/ls/host.ts package.json
• pnpm build

Closes #11