fix: resolve requested channels directly for targeted archived thread sync

[steipete]

Rebased and completed from #120 by @robinbraemer.

Targeted archived-thread sync now resolves every requested channel directly with GET /channels/{id} before broad thread-catalog discovery. This preserves the targeted path even when archived thread discovery cannot find the requested channel globally.

The completed implementation handles requested channels owned by multiple guilds, performs only one direct lookup per requested channel, rejects DMs from the guild archived-thread path, and adds regression coverage for direct resolution, lookup caching, multi-guild ownership, and DM rejection.

This PR supersedes #120 and closes it while preserving @robinbraemer's authorship credit in the squash commit.

[clawsweeper]

ClawSweeper status: review started.

I am starting a fresh review of this pull request: fix: resolve requested channels directly for targeted archived thread sync This is item 1/1 in the current shard. Shard 0/1.

This placeholder means the worker is alive and reading the current context. I will edit this same comment with the actual review when the claws are done clicking.

Crustacean status: shell secured, claws on keyboard, evidence pebbles being sorted.