fix(discord): full thread support with session isolation and allowlist enforcement

Discord threads use their own channel IDs, so allowChannels was blocking
thread replies unless each thread ID was listed explicitly.

- Include the thread parent channel ID as an allowlist candidate
- Enforce allow_channels on slash commands (previously bypassed)
- Show parent channel ID in runtime context, reply to the thread
- Fix subagent cancel key via effective_key propagation
- Detect bot mentions via raw_mentions and reply-to-bot references
- Cache seen thread channels for outbound delivery
- Ignore system messages that become empty prompts

docs/chat-apps.md

@@ -147,7 +147,7 @@ If you prefer to configure manually, add the following to `~/.nanobot/config.jso
 > - `"open"` — Respond to all messages
 > DMs always respond when the sender is in `allowFrom`.
 > - If you set group policy to open create new threads as private threads and then @ the bot into it. Otherwise the thread itself and the channel in which you spawned it will spawn a bot session.
-> `allowChannels` restricts the bot to specific Discord channel IDs. Empty (default) means respond in every channel the bot can see. Example: `["1234567890", "0987654321"]`. The filter applies after `allowFrom`, so both must pass.
+> `allowChannels` restricts the bot to specific Discord channel IDs. Empty (default) means respond in every channel the bot can see. Example: `["1234567890", "0987654321"]`. The filter applies after `allowFrom`, so both must pass. Discord threads under an allowed parent channel are also allowed; for Forum channels, allowing the parent Forum channel allows all threads/posts in that forum.
 > `streaming` defaults to `true`. Disable it only if you explicitly want non-streaming replies.
 
 **5. Invite the bot**
@@ -658,4 +658,4 @@ Create or reuse a Microsoft Teams / Azure bot app registration. Set the bot mess
 nanobot gateway
 ```
 
-</details>
+</details>