Extensions-move cleanup

Upstream commits:

• 439c21e078 — Remove channel shim directories, point all imports to extensions (~534 files)
• ac29edf6c3 — Update vitest configs after channel move (~19 files)

Parent issue: Replaces part of #2018

Prerequisites

All 7 adapter move issues must land first (Mattermost, Signal, iMessage, Slack, Telegram, Discord, WhatsApp).

Exit Criteria

• pnpm typecheck passes
• pnpm test passes
• No stale shim directories remain

Decomposed from #2018