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>

internal/cli/agent.go

@@ -27,6 +27,7 @@ type Mailer interface {
 type Deps struct {
 	Store *store.Store
 	Dial  func(store.Account) (Mailer, error)
+	Send  func(store.Account, mail.OutgoingMessage) error
 	Now   func() time.Time
 	Out   io.Writer
 }