Extensions-move: iMessage adapter

Upstream commits (all 3 required for typecheck):

1. 0ce23dc62d — Move iMessage channel to extensions/imessage (~53 files)
2. c1ef5748eb — Enforce scoped plugin sdk imports (~14 files) — partial pick: iMessage-related only
3. a622eecd3b — Dedupe shared test helpers (~45 files) — partial pick: iMessage-related only

Decomposed from: #2018

Why all 3 are needed

The move commit introduces files with import paths that don't resolve until the follow-up commits land. Without them, pnpm typecheck fails. Same pattern as WhatsApp move (#2031/#2035).

Conflict Resolution

Move src/imessage/ → extensions/imessage/src/. Keep fork's rebranded content. For commits 2 and 3, only pick iMessage-related hunks — skip provider/GUT-layer changes.

Exit Criteria

• pnpm typecheck passes
• No openclaw references in moved files
• src/imessage/ contains only re-export shims (or is empty)