Add AgentMail plugin: paid-member mailbox client for humans + agents

@logicsrc/plugin-agentmail provides read/search/compose/send/flag/delete
over an injected MailTransport, gated to Founding Lifetime (paid) members.
Returns plain JSON-serializable domain objects so the same API serves a
human TUI, the CLI, MCP, and bots.

- domain.ts: transport-agnostic types + pure helpers (parse/format address,
  normalizeDraft, isValidEmail, snippet)
- ports.ts: MailTransport seam
- service.ts: AgentMailService (inbox/list/read/search/send/reply/flag/delete)
- access.ts: paid-member gate (assertPaid) + capability constants
- transports/memory.ts: complete in-memory backend (tests/dev/reference)
- transports/mailu.ts: self-hosted Mailu seam (mail.profullstack.com IMAP +
  smtp.profullstack.com submission) with injected IMAP/SMTP drivers
- index.ts: PluginDefinition (manifest, routes, capabilities, tuiPanels)
- registered in the root build; 20 vitest tests pass

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

plugins/agentmail/src/index.ts

@@ -0,0 +1,40 @@
+import type { PluginDefinition } from "@logicsrc/plugin-core";
+import { agentMailManifest } from "./manifest.js";
+
+export const agentMailPlugin: PluginDefinition = {
+  manifest: agentMailManifest,
+  configDefaults: {
+    enabled: false,
+    members_only: true,
+    paid_only: true,
+    domain: "${AGENTMAIL_DOMAIN}",
+    imap_host: "${AGENTMAIL_IMAP_HOST}",
+    imap_port: "${AGENTMAIL_IMAP_PORT}",
+    smtp_host: "${AGENTMAIL_SMTP_HOST}",
+    smtp_port: "${AGENTMAIL_SMTP_PORT}",
+    backend_provider: "mailu"
+  },
+  routes: [
+    { method: "GET", path: "/api/plugins/agentmail/mailboxes", capability: "mailbox.list" },
+    { method: "GET", path: "/api/plugins/agentmail/mailboxes/:mailbox/messages", capability: "message.list" },
+    { method: "GET", path: "/api/plugins/agentmail/mailboxes/:mailbox/messages/:uid", capability: "message.read" },
+    { method: "GET", path: "/api/plugins/agentmail/search", capability: "message.search" },
+    { method: "POST", path: "/api/plugins/agentmail/messages", capability: "message.send" },
+    { method: "PATCH", path: "/api/plugins/agentmail/mailboxes/:mailbox/messages/:uid", capability: "message.flag" },
+    { method: "DELETE", path: "/api/plugins/agentmail/mailboxes/:mailbox/messages/:uid", capability: "message.delete" }
+  ],
+  permissions: ["mail:read", "mail:search", "mail:compose", "mail:send", "mail:flag", "mail:delete"],
+  tuiPanels: [{ id: "agentmail-inbox", title: "Mail" }]
+};
+
+export { agentMailManifest };
+
+// Domain, access, ports, service, and transports.
+export * from "./domain.js";
+export * from "./access.js";
+export * from "./ports.js";
+export * from "./service.js";
+export { InMemoryMailTransport } from "./transports/memory.js";
+export type { SeedMessage } from "./transports/memory.js";
+export { createMailuTransport, resolveMailuConfig } from "./transports/mailu.js";
+export type { MailuConfig, ImapDriver, SmtpDriver, CreateMailuTransportOptions } from "./transports/mailu.js";