Fix Telegram group activation and improve documentation

Problem

1. /activation always command appeared to work but had no effect
2. Documentation didn't clearly explain group access control
3. Users confused about telegram.groups creating an allowlist
4. allowFrom file was incorrectly used for groups

Root Cause

• Bot filtered messages based on config requireMention setting only
• Session state (updated by /activation) was never consulted
• This created a broken UX where the command seemed to succeed but didn't work

Solution

Code Fix (src/telegram/bot.ts)

• Load session state early to check groupActivation field
• Check session state before falling back to config
• Makes /activation always and /activation mention commands actually work
• Respects forum topics by including messageThreadId in session key

Documentation (docs/providers/telegram.md)

• Add detailed "Group activation modes" section
• Clarify two-level access control:

1. Group allowlist (telegram.groups)
2. Sender filtering (telegram.groupPolicy)

• Document /activation command properly
• Add troubleshooting section with common issues
• Explain how to get group chat ID

Testing

• x Group with requireMention: false in config responds to all messages
• x /activation always enables always-respond mode
• x /activation mention enables mention-only mode
• x Settings persist across gateway restarts (stored in session)
• x Wildcard "*" in telegram.groups works correctly
• x Group not in allowlist is blocked when groups config exists

Migration Impact

• Backwards compatible: Existing behavior
• unchanged
• /activation command that was broken will now work
• No config changes required