v0.1.1

Improve agent-facing diagnostics from black-box testing findings

- wait/otp timeouts now explain near-misses: mail predating the --since
  anchor (with sender and gap), mail excluded by the snapshot baseline,
  and mail present but failing the --from/--subject filters. Exit code
  and JSON error shape are unchanged.
- Passing an inbox id or address to --handle now names the mistake
  (confirming against the local store when possible) and points at
  positional usage / `tmail new --stateless`, instead of a bare
  "invalid handle blob".
- Clarify --since and --handle help text; update DESIGN.md §4 to match
  the created_at-seeded default baseline.
- Bump to 0.1.1.