Vouch request: fix for deeply nested subagent agent:// resolution (#4650)

[any-victor]

Hey @can1357 — following up on the Discord ping.

A couple of the bugs I logged already got merged (#4649, #4648), but #4650 still needs a vouch before I can PR the fix, which I've already got staged and ready: any-victor#1

It's a small one — agent:// can't resolve a deeply nested subagent's output (depth ≥ 2) because task/index.ts writes each level one dir deeper than registry-helpers.ts looks, so artifactsDirsFromRegistry just needs to check both candidate dirs instead of one-or-the-other. Regression test included, fails on main and passes with the fix, bun check green.

Would really appreciate the vouch so I can open it. Cheers!

[can1357]

!vouch @any-victor
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