feat(send): Phase 2 send path — SMTP, MIME, reply threading, outbound policy

Adds the `send` agent command and everything behind it:

- store: Account carries SMTP host/port/security (NULL-safe scan/insert/select);
  admin `account add` gains --smtp-* flags (applied for RW accounts).
- policy: OutboundRule.Check(recipients) → (ok, reason); RO ⇒ ro_mode,
  whitelist-out blocks the whole send if any recipient fails (no partial send).
- mail: Header.References; OutgoingMessage + BuildMIME (plain text + attachments,
  In-Reply-To/References threading, Bcc envelope-only); SendSMTP (tls/starttls,
  SASL PLAIN, envelope send) via emersion/go-smtp.
- cli: SendCmd gates outbound, resolves --reply-to under the inbound filter
  (filtered/absent source ⇒ not_found), reads attachments, audits, emits the
  JSON envelope; repeatable --to/--cc/--bcc/--attach flags wired into the router.

Implemented test-first; full suite passes incl -race. Validated live against
friday.mxlogin.com: real send to me@stevecliff.com, RO + whitelist-out blocks,
and --reply-to threading off a live INBOX message. test-creds.md gitignored.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

go.mod

@@ -5,12 +5,13 @@ go 1.25.0
 require (
 	github.com/emersion/go-imap v1.2.1
 	github.com/emersion/go-message v0.18.2
+	github.com/emersion/go-sasl v0.0.0-20241020182733-b788ff22d5a6
+	github.com/emersion/go-smtp v0.24.0
 	modernc.org/sqlite v1.53.0
 )
 
 require (
 	github.com/dustin/go-humanize v1.0.1 // indirect
-	github.com/emersion/go-sasl v0.0.0-20200509203442-7bfe0ed36a21 // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/ncruces/go-strftime v1.0.0 // indirect