Implement email support with per-user permission model (closes #2)

New modules:
- server/mailer.mjs: nodemailer transport wrapper
- server/mail_perms.mjs: runtime permission store, persisted to disk

New actions:
- send-email: checks (caller, to, topic) permission before sending
- set-mail-permission: grant/revoke permissions, gated by canApprove
- get-mail-permissions: list current permissions

Handler signature extended to handler(params, ctx) where ctx carries
caller, users, mail_perm_store and mailer_send. Existing handlers
ignore ctx so the change is backwards-compatible.

SMTP config lives in secrets.json under optional 'smtp' key.
Mail permissions path via --mail-perms or CONDUIT_MAIL_PERMS.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

secrets.example.json

@@ -1,4 +1,11 @@
 {
+	"smtp": {
+		"host": "smtp.example.com",
+		"port": 587,
+		"secure": false,
+		"auth": { "user": "relay@example.com", "pass": "<password>" },
+		"from": "agent@example.com"
+	},
 	"users": {
 		"<username>": { "secret": "<hex secret>", "canApprove": [] },
 		"<username>": { "secret": "<hex secret>", "canApprove": ["<username>"] }