Rewrite as a Rust, Apache-2.0 workspace

Supersedes the Go + embed-Mox design. The Go tree is removed; its
architecture doc is preserved at docs/archive/ARCHITECTURE-go-embed-mox.md
because its competitive analysis and data model still hold.

Five decisions recorded as ADRs:

  0001  Rust, not Go — accepting ~5,500 lines of protocol code that Mox
        would have given us free, to get the first permissively licensed
        Rust mail server. Costs stated plainly.
  0002  Apache-2.0, not MIT or AGPL — patent grant, trademark, CLA-free
        contribution. Public on GitHub; Gitea stays as the private fallback.
  0003  Stalwart's primitive crates (Apache-2.0/MIT) yes; its AGPL server
        crates never. DANE and MTA-STS sit on the AGPL side of that line,
        which is why we write our own.
  0004  Milestones, reordered: embedded inbound is required at launch.
  0005  Oracle Cloud blocks outbound :25, so direct-to-MX is impossible on
        the launch host. Split delivery is mandatory, not an on-ramp.

Twelve crates in three tiers. Tier 1 (mail-dane, mail-mta-sts, mail-dsn)
is standalone and publishable — no `dane` or `mta-sts` crate exists on
crates.io at all today.

openmail-relay ships the provider table as data, with SES and Oracle from
the start. Oracle's and Resend's SPF includes are deliberately None: a
guessed include turns the DNS check green against a mechanism the provider
does not honour, and mail still fails SPF silently.

cargo check/test/clippy/fmt all green; unsafe_code is forbidden workspace
wide; cargo-deny enforces the licence policy in CI.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JkyvfNJGTshJNE9FtwPLk7

SECURITY.md

@@ -0,0 +1,31 @@
+# Security policy
+
+OpenMail runs a parser on port 25, exposed to the open internet, with its
+source published. That is the same position Postfix and Mox are in, and it is
+safe only with a real disclosure process. This is ours.
+
+## Reporting
+
+**Do not open a public issue for a security bug.**
+
+Use GitHub's [private vulnerability reporting](https://github.com/karti-ai/openmail/security/advisories/new),
+or email the maintainer. We will acknowledge within 72 hours.
+
+## Scope — what we consider a vulnerability
+
+- Anything reachable pre-authentication on the SMTP listener.
+- MIME parsing that panics, hangs, or allocates unboundedly on crafted input.
+- **A silent downgrade of a security property**: DANE or MTA-STS reporting
+  success where the policy was not actually satisfied, or a policy that should
+  have been enforced being skipped. These are the highest-severity class in
+  this codebase precisely because they do not look like failures.
+- Cross-tenant (`pod`) data access.
+- Authentication or scope bypass in the REST or MCP surfaces — especially an
+  MCP tool reaching a credential route (see `openmail_mcp::Exposure`).
+
+## Not in scope
+
+- Deliverability problems (mail landing in spam).
+- Missing rate limits on an endpoint behind authentication, unless it is
+  amplification.
+- Reports from automated scanners with no demonstrated impact.