fix(gateway): preserve thread routing for /update progress and prompts

[teknium1]

Salvage of #17465 (@Yukipukii1) onto current main.

Summary

/update now keeps progress streams, interactive prompts, timeout notices, and post-restart completion notifications in the originating thread/topic. Previously only chat_id was persisted, so every /update reply fell back to the parent chat when invoked from a Telegram topic, Discord thread, or Slack thread.

Changes

• gateway/run.py: persist thread_id in .update_pending.json; thread metadata={"thread_id": ...} through the watcher's stream/prompt/timeout/final-notification sends
• gateway/platforms/telegram.py: send_update_prompt() honors message_thread_id
• gateway/platforms/discord.py: send_update_prompt() targets the thread channel when present
• Slack needs no adapter change — its existing send() already resolves metadata.thread_id to a thread_ts
• Regression coverage in tests/gateway/test_update_command.py and tests/gateway/test_update_streaming.py

Validation

• tests/gateway/test_update_command.py + tests/gateway/test_update_streaming.py — 48/48 passed
• Verified on current main: zero thread_id references anywhere in the /update pending/watcher paths, so the reported routing loss is real

Closes #17465. Contributor authorship preserved via cherry-pick + rebase-merge.