fix(imap): use native MOVE and targeted fallback expunge

[bscott]

Problem

MoveMessages implemented moves as COPY + STORE \Deleted + an unqualified EXPUNGE. EXPUNGE removes every deleted message in the selected mailbox, so moving one message could permanently delete unrelated messages previously marked \Deleted.

Approach

• Delegate moves to go-imap v2 Client.Move, which uses native RFC 6851 MOVE when available.
• Preserve no-match detection by sharing the UIDPLUS response-data check between COPY and MOVE.
• Exercise the library fallback without MOVE; UID selectors on UIDPLUS servers use targeted UID EXPUNGE.
• Document the fix under Unreleased.

Verification

• gofmt -l . (clean)
• go vet ./...
• go test ./...
• Regression tests were sabotaged against the old implementation and both failed as expected.
• Independent review: Claude Opus 4.6 — APPROVE, no blocking findings.

Risk

Low. The implementation removes custom protocol sequencing in favor of the dependency’s built-in native MOVE/fallback behavior. Existing successful, missing-UID, UIDPLUS, and non-UIDPLUS tests remain green.

Closes #28
Closes #29