feat(discord): smart auto-thread mode

[Hypn0sis]

Summary

• Adds auto_thread config field to DiscordConfig with three modes:
  • "true" — always create a thread from each message
  • "false" — never create a thread (default, opt-in)
  • "smart" — create a thread only when the bot is @mentioned
• Thread name is derived from the message content (strips mention prefix, truncates to Discord's 100-char limit)
• Auto-thread creation happens after all policy guards (group_policy, mention_only, free_response, rate-limit) so threads are never created for messages the bot would ignore
• Incoming messages from inside a bot-created thread are detected and routed back into the same thread, preventing duplicate threads
• MESSAGE_UPDATE events (Discord embed resolution) are deduplicated against already-forwarded MESSAGE_CREATE events to prevent double responses
• Fixes api_send_thread_message to POST directly to channels/{thread_id}/messages (the correct Discord API endpoint for threads)
• Adds RevoltAdapter::with_urls_and_channels constructor to support custom URLs + channel restrictions together

Config example

[channels.discord]
bot_token_env = "DISCORD_BOT_TOKEN"
auto_thread = "smart"   # or "true" / "false"

Test plan

• cargo check --workspace --lib passes
• cargo clippy -p openfang-channels -p openfang-types -p openfang-api -- -D warnings passes
• cargo test -p openfang-channels -p openfang-types passes
• Live: set auto_thread = "smart", mention bot in a server channel → thread created
• Live: set auto_thread = "smart", message without mention → no thread
• Live: set auto_thread = "false" → no threads ever
• Live: replies inside a bot-created thread stay in the thread (no new thread spawned)

[Hypn0sis]

Closing in favour of two separate PRs: fix(deps) CVEs → #1041, feat(discord) smart auto-thread → new PR.