fix: empty allowed_channels denies all channels (secure by default)

[chaodu-agent]

Summary

Implements Option A from #91: empty allowed_channels now means the bot will not respond to any messages (secure by default).

Changes

src/discord.rs

• Removed self.allowed_channels.is_empty() || from the channel check — empty set now denies all
• Added startup warning log in ready() when allowed_channels is empty:

  WARN: allowed_channels is empty — bot will NOT respond to any messages. Configure allowed_channels in config.toml.
  

config.toml.example

• Added comment: # Required: at least one channel ID. Empty list = bot will not respond to any messages.

Before / After

Scenario	Before	After
allowed_channels = []	✅ Responds in ALL channels	❌ Responds in NO channels + warning log
allowed_channels = ["123"]	✅ Responds in channel 123	✅ Responds in channel 123 (unchanged)
Thread in allowed channel	✅ Works	✅ Works (unchanged)

Breaking Change

Users who intentionally left allowed_channels empty to allow all channels will need to explicitly list their channel IDs. This is a one-line config change.

Closes #91

[masami-agent]

Clean change, well-scoped 👍 Two things to address:

1. Needs rebase on main

PR #108 (allowed_users) has since merged and modified the same area of discord.rs. This PR will likely have conflicts. The allowed_users check also needs the same treatment — after rebase, please verify the user check logic still works correctly alongside this change.

2. Behavioral consistency with allowed_users

After this PR, the semantics would be:

• allowed_channels = [] → deny all (this PR)
• allowed_users = [] → allow all (#108)

This asymmetry could confuse users. Worth discussing: should allowed_users = [] also mean deny all? Or is the current design intentional (channels = required, users = optional filter)?

If intentional, please add a note in the PR description and config.toml.example clarifying the difference. If not, consider aligning the behavior.

Otherwise LGTM — the secure-by-default approach for channels makes sense.