v2.2.0 — add mailbox (IMAP+SMTP gateway)

New service psyb0t/docker-mailbox v0.3.0 at /mailbox/. Stateless
gateway across N email accounts from a single YAML config — unified
inbox, per-account list/search/CRUD, SMTP send. No DB; every read
hits upstream IMAP live.

Direct route only (not in LiteLLM providers). MCP enabled by default
with 9 flat tools via the /mcp/ aggregator — per-account tools take
`mailbox` as a parameter, catalog stays flat regardless of inbox count.

Bearer-token auth (MAILBOX_AUTH_TOKEN). Config file holds plaintext
creds — MAILBOX_CONFIG must point at a gitignored host path; template
at mailbox/config.example.yaml; MAILBOX_CONFIG added to Makefile's
_FILE_VARS so the stack refuses to start without it.

tests/test_mailbox.sh: 4 always-on tests + 1 opt-in e2e test
(send→recv→read→delete) gated on MAILBOX_TEST_MAILBOX_NAME +
MAILBOX_TEST_ADDRESS. All gated on MAILBOX=1.

Co-shipped with v2.1.0 (predictalot) on the same commit — shared
edits to docker-compose, .env.example, Makefile, README, docs.

Also fixes dangling .data/pibox-zai/ .gitkeep allowlist from v2.0.0.