feat(plugins): Vision Pack v0 — full-screen screenshot for AI fallback

[rrader26]

Summary

• New `createVisionPlugin()` adds `agentmark_screenshot` for full-screen capture.
• Layer 2 of the desktop driver stack: when accessibility is too sparse to drive an app, the multimodal AI parses the screenshot directly.
• Cross-platform pure-Node (shells out to OS-native screenshot CLIs).

Why

Closes the accessibility-coverage gap from the strategic discussion: 99% of well-designed apps expose a usable a11y tree, but custom-drawn UIs, weak-a11y Electron apps, and games don't. Screenshot + multimodal vision = universal fallback. No per-app engineering needed — Claude's own vision handles parsing.

What's in v0

Surface	What
Capture	Full-screen PNG, inline base64 OR write to disk
Platforms	macOS `screencapture`, Windows PowerShell + .NET, Linux `gnome-screenshot`/`scrot`
Output	`{ image_base64?, path?, bytes, captured_by, platform }`

What's NOT in v0 (deliberate)

• Per-window screenshots — needs bridge work (UIA `GraphicsCaptureItem` / macOS `CGWindowListCreateImage`). Follow-up PR after window-management lands.
• OmniParser-style structured GUI parsing — the AI does its own parsing for now; structured parsing inside the plugin can come later as a model integration.
• Auto-fallback hook on `agentmark_desktop_snapshot` when the tree is sparse — needs a heuristic ("element_count < N, role-coverage shallow") + a clean control-flow path in the desktop plugin.

Usage

```ts
import { createMcpServer, createDesktopPlugin, createVisionPlugin } from '@thinkfleet/agentmark'

const vision = createVisionPlugin()
createMcpServer({ plugins: [desktop, vision, meta] })
```

When the agent gets an empty / sparse snapshot, it can fall back:
```
agentmark_screenshot { output_path: "/tmp/fallback.png" }
→ multimodal AI vision-parses the image with its native vision model
```

Test plan

• `pnpm build` clean
• `pnpm test` — 345 pass / 10 skip (3 new: plugin registration)
• Manual on macOS: `agentmark_screenshot` returns a valid PNG
• Manual on Windows: same; verify PowerShell .NET drawing path works on Win 11 ARM64

🤖 Generated with Claude Code