Fail closed on empty channel allowlists

[lawyered0]

Summary

This PR closes a critical channel-authentication gap by requiring explicit sender allowlists by default.

Critical issue addressed

Telegram and Discord startup previously treated empty allowFrom as open mode, allowing any external user to interact with the bot when enabled.

What changed

• Added an explicit environment gate in internal/channels/security.go:
  • PICOBOT_ALLOW_PUBLIC_CHANNELS=1 is now required to opt into public/open channel mode.
• Hardened Telegram startup in internal/channels/telegram.go:
  • startup now fails when allowFrom is empty unless public mode is explicitly enabled.
• Hardened Discord startup in internal/channels/discord.go:
  • startup now fails when allowFrom is empty unless public mode is explicitly enabled.
• Added regression tests:
  • internal/channels/telegram_test.go (reject empty allowFrom by default)
  • internal/channels/discord_test.go (reject empty allowFrom by default)
  • internal/channels/security_test.go (env gate behavior)

Validation

• go test ./internal/channels -count=1
• go test ./...

Compatibility note

Users relying on implicit open mode must now explicitly set PICOBOT_ALLOW_PUBLIC_CHANNELS=1.