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/package.json

@@ -0,0 +1,18 @@
+{
+  "name": "@logicsrc/plugin-agentmail",
+  "version": "0.1.0",
+  "description": "Agent-native mail: a paid-member mailbox client (read/search/compose/send) over an injected IMAP/SMTP transport, designed for both bots and humans.",
+  "type": "module",
+  "main": "./dist/index.js",
+  "types": "./dist/index.d.ts",
+  "scripts": {
+    "build": "tsc -p tsconfig.json",
+    "test": "vitest run src --passWithNoTests"
+  },
+  "dependencies": {
+    "@logicsrc/plugin-core": "file:../../packages/plugin-core"
+  },
+  "devDependencies": {
+    "vitest": "^4.0.8"
+  }
+}