feat: add beepctl react command

[blqke]

Summary

• add a first-class beepctl react command for adding and removing reactions
• share the write-permission error hint between send and react
• document the command, add focused tests, and bump the package to 0.1.6

Verification

• pnpm build
• pnpm exec vitest run tests/react-command.test.ts
• pnpm exec vitest run tests/client.test.ts
• node dist/cli.js react --help
• pnpm lint
• pnpm typecheck

Notes

• upgraded @beeper/desktop-api to 4.7.0 because the previously pinned SDK version did not expose the official client.chats.messages.reactions.* methods
• live-validated add/remove against a Note to self message in Beeper Desktop