Record passive group context

[pmbstyle]

Summary

• Record non-addressed group messages as passive group observations instead of dropping them from memory.
• Preserve those observations in shared recent history as system context, without treating them as direct user turns.
• Let Telegram observe group messages from other bots, so agents can use each other's group replies as context without responding to them.

Why

Agents in a shared group should understand the group conversation even when a message was not addressed to them. Previously the group gate returned before writing memory, and Telegram dropped bot-authored group messages entirely, so other agents could not see a peer's reply as context.

Validation

• uv run ruff check src/octopal/channels/group_observation.py src/octopal/channels/telegram/handlers.py src/octopal/channels/whatsapp/runtime.py src/octopal/runtime/memory/service.py tests/test_memory_system.py tests/test_telegram_react_tag_sanitization.py
• uv run pytest