[1.3] Clipboard screenshot paste support

[dotnetspark]

Implement cross-platform clipboard image capture:

• Windows: PowerShell + System.Windows.Forms.Clipboard -> temp PNG
• macOS: osascript + NSPasteboard -> temp PNG
• Linux: xclip/xsel fallback

Acceptance Criteria

• read_clipboard_image MCP tool returns base64 PNG on all 3 platforms
• Unsupported platform returns informative error
• clipboard.test.ts passes (mocked child_process)
• Agent triggers clipboard read on natural language cues

See IMPLEMENTATION_PLAN.md milestone 1.3

[github-actions]

📋 Assigned to Zoe (Core Dev)

Issue: #3 — [1.3] Clipboard screenshot paste support

Zoe will pick this up in the next Copilot session.

For Copilot coding agent: If enabled, this issue will be worked automatically.
Otherwise, start a Copilot session and say:
Zoe, work on issue #3

[github-actions]

🏗️ Squad Triage — Mal (Lead/Architect)

Issue: #3 — [1.3] Clipboard screenshot paste support
Assigned to: Simon (QA Engineer)
Reason: Issue relates to testing/quality work

Team roster:

• Mal (Lead/Architect) → label: squad:mal
• Zoe (Core Dev) → label: squad:zoe
• Kaylee (UX Engineer) → label: squad:kaylee
• Simon (QA Engineer) → label: squad:simon
• Book (Docs/DevRel) → label: squad:book
• Ralph (Work Monitor) → label: squad:ralph

To reassign, remove the current squad:* label and add the correct one.