fix: allow direct sends without Messages DB access

[steipete]

Summary

• keep direct imsg send --service auto working when chat.db is not readable
• make local service detection best-effort for direct recipients while preserving required DB access for chat-target sends
• document the Full Disk Access fallback behavior and add a regression test

Proof

• autoreview clean
• live e2e: swift run imsg send --to '+436769770569' --text 'imsg e2e test 2026-05-31T18:28:50Z' --service auto --region AT --json -> {"status":"sent"}
• make lint
• make test (321 tests)

[clawsweeper]

ClawSweeper status: review started.

I am starting a fresh review of this pull request: fix: allow direct sends without Messages DB access 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.