Summary

This PR adds opt-in Telegram topic-based routing so forum threads can keep isolated session/memory context.

Closes #220.

Problem

In Telegram supergroups with forum topics, MicroClaw previously keyed Telegram sessions only by chat_id.
That merged all topics in the same group into one conversation state.

User impact:

• cross-topic context leakage
• "agent yapping" across unrelated topic workspaces
• harder multi-agent workflows in one Telegram hub

Root Cause

external_chat_id for Telegram was always the raw Telegram chat_id, and message_thread_id was ignored for session routing.

Changes

1. Add Telegram config:

• channels.telegram.topic_routing.enabled (default false)

2. Route Telegram session identity by topic when enabled:

• Build external_chat_id as {chat_id}:{thread_id} when message_thread_id exists and routing is enabled
• Keep legacy {chat_id} behavior when disabled or when no thread id is present

3. Keep Telegram outbound compatibility for topic-scoped chats:

• Teach Telegram adapter to parse both external_chat_id shapes:
  • {chat_id}
  • {chat_id}:{thread_id}
• When thread id is present, send responses/attachments with message_thread_id

4. Tests:

• runtime context default/off behavior assertions
• runtime context enabled parsing assertion
• external chat id generation test for enabled/disabled/no-thread cases
• updated existing send-message test fixture for new Telegram config field

Validation

Executed:

• cargo test -p microclaw --lib channels::telegram::tests::test_build_telegram_runtime_contexts_topic_routing_enabled -- --nocapture
• cargo test -p microclaw --lib channels::telegram::tests::test_telegram_external_chat_id_topic_routing -- --nocapture
• cargo test -p microclaw --lib tools::send_message::tests::test_send_message_web_caller_cross_chat_denied -- --nocapture

All passed.

Topic Routing Config Priority

Telegram topic routing now supports both channel-level default and per-account override.

Priority order:

1. channels.telegram.accounts.<account_id>.topic_routing.enabled (highest)
2. channels.telegram.topic_routing.enabled
3. false (implicit default)

This allows gradual rollout by account while keeping backward compatibility.

Migration Examples

Channel-wide enable:

channels:
  telegram:
    topic_routing:
      enabled: true

Mixed mode (channel default + account override):

channels:
  telegram:
    topic_routing:
      enabled: false
    accounts:
      main:
        enabled: true
        bot_token: "..."
        bot_username: "..."
      ops:
        enabled: true
        bot_token: "..."
        topic_routing:
          enabled: true

In the mixed mode above:

• main follows channel default (false)
• ops uses account override (true)