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/README.md

@@ -0,0 +1,62 @@
+# @logicsrc/plugin-agentmail
+
+Agent-native mail for LogicSRC: a paid-member mailbox client (read, search,
+compose, send, flag, delete) that works the same for **humans** (a TUI) and
+**bots/agents** (the CLI, MCP, or direct service calls), because every method
+returns plain JSON-serializable domain objects.
+
+## Design
+
+The plugin follows the LogicSRC plugin pattern (cf. `agentgit`):
+
+- **`domain.ts`** — transport-agnostic types (`Mailbox`, `MessageSummary`,
+  `Message`, `Draft`, `MailAddress`) and pure helpers (`parseAddress`,
+  `formatAddress`, `normalizeDraft`, `isValidEmail`, `snippet`).
+- **`ports.ts`** — the `MailTransport` seam. The service talks only to this.
+- **`service.ts`** — `AgentMailService`: paid-gated, ergonomic operations
+  (`inbox`, `list`, `read`, `search`, `send`, `reply`, `flag`, `delete`).
+- **`access.ts`** — mail is a **Founding Lifetime Member** (paid) perk; every
+  call runs `assertPaid`.
+- **`transports/memory.ts`** — `InMemoryMailTransport`: a complete, dependency
+  free backend for tests, local dev, and as the reference implementation.
+- **`transports/mailu.ts`** — the self-hosted Mailu seam
+  (`mail.profullstack.com` IMAP + `smtp.profullstack.com` submission). It takes
+  **injected** IMAP/SMTP drivers so heavy network libraries stay out of the
+  plugin; a consuming app supplies the concrete drivers (e.g. `imapflow` +
+  `nodemailer` in Node, or the Go client on the BBS).
+
+## Usage
+
+```ts
+import { AgentMailService, InMemoryMailTransport } from "@logicsrc/plugin-agentmail";
+
+const transport = new InMemoryMailTransport();
+const mail = new AgentMailService({
+  transport,
+  identity: { name: "alice", paid: true },
+  domain: "mail.profullstack.com"
+});
+
+await mail.send({ to: [{ address: "carol@example.com" }], subject: "Hi", text: "hello" });
+const inbox = await mail.inbox();
+```
+
+Against the real stack:
+
+```ts
+import { AgentMailService, createMailuTransport, resolveMailuConfig } from "@logicsrc/plugin-agentmail";
+
+const config = resolveMailuConfig({ user: "alice", pass: process.env.MAIL_PASS! });
+const transport = createMailuTransport({ config, imap, smtp }); // imap/smtp: app-provided drivers
+const mail = new AgentMailService({ transport, identity: { name: "alice", paid: true }, domain: config.domain });
+```
+
+## Config / env
+
+| Var | Default | Meaning |
+|---|---|---|
+| `AGENTMAIL_DOMAIN` | `mail.profullstack.com` | member address domain |
+| `AGENTMAIL_IMAP_HOST` | `mail.profullstack.com` | IMAP host |
+| `AGENTMAIL_IMAP_PORT` | `993` | IMAPS port |
+| `AGENTMAIL_SMTP_HOST` | `smtp.profullstack.com` | SMTP submission host |
+| `AGENTMAIL_SMTP_PORT` | `587` | submission port (STARTTLS) |