refactor(mcp): plugin registry + AgentMarkPlugin contract

[rrader26]

Summary

• Introduce AgentMarkPlugin contract and Dispatcher registry class.
• Carve web / pdf / desktop / meta capabilities into plugin factories.
• dispatcher.ts / tool-defs.ts / server.ts become thin shims over the registry; legacy functional API (dispatch, createDispatcherState, disposeAll, ALL_TOOLS) is preserved unchanged.
• createMcpServer({ plugins?: AgentMarkPlugin[] }) now accepts a custom plugin set.

Why

Foundation for the Microsoft Workflows Pack (and Insurance, etc.) — external packs need a first-class registration API, not a special-cased extra case in the dispatcher switch. Going with the registry refactor rather than a bolt-on so future packs are first-class.

What stayed identical

• Every existing test passes unchanged (317 prior + 9 new plugin-contract tests = 326 passing).
• All public exports (functional API + types) keep their names and call signatures.
• Tool names and behavior are byte-identical — the wire format is unchanged.

Test plan

• pnpm build clean
• pnpm test — 326 pass, 10 skip (same skip set as main)
• Manual: spawn agentmark-mcp from a fresh Claude Code session on Windows VM, run agentmark_desktop_list_targets, confirm same window enumeration as before refactor
• Manual: same on macOS against TextEdit

New public API

From @thinkfleet/agentmark (or the /mcp subpath):

• AgentMarkPlugin, ToolHandler, DispatchResult — author surface
• Dispatcher — registry/router class
• createWebPlugin, createPdfPlugin, createDesktopPlugin, createMetaPlugin — first-party factories

Example pack:
```ts
import { type AgentMarkPlugin } from '@thinkfleet/agentmark'

export function createMicrosoftPlugin(opts): AgentMarkPlugin {
return {
name: 'microsoft',
tools: [/* MS-specific tool defs /],
handlers: { / keyed by tool name / },
dispose: async () => { / cleanup */ },
}
}
```

🤖 Generated with Claude Code