feat(co-browser): Create instruction panel with markdown rendering

[coderooz]

Create the instruction panel state manager for the co-browser system.

Acceptance criteria:

• Create src/domains/browser/instruction-panel.js
• Store instructions per-window (global) and per-tab
• Agents write instructions via MCP tool
• Plain text with markdown-like formatting (bold, lists, code blocks)
• Instructions persist across page navigation (static panel)
• Support instruction update, clear, and query
• Simple markdown rendering (regex-based, no external deps)

Interface:

• setInstructions(windowId, instructions, tabId?) → success
• getInstructions(windowId, tabId?) → formatted instructions
• clearInstructions(windowId, tabId?) → success
• renderMarkdown(text) → HTML string

Milestone: v2.8.0