Add cli/ directory with standalone HTTP + WebSocket server that serves
the pixel office in a regular browser window, without requiring VS Code.

Architecture:
- cli/main.ts — entry point, parses args, opens browser
- cli/server.ts — HTTP static file server + WebSocket on /ws
- cli/cliOrchestrator.ts — replaces PixelAgentsViewProvider, broadcasts via WS
- cli/sessionScanner.ts — auto-detects Claude Code sessions in ~/.claude/projects/
- cli/persistence.ts — seats/settings stored in ~/.pixel-agents/
- cli/vscode-stub.ts — minimal vscode API shim for bundling

Frontend changes:
- webview-ui/src/vscodeApi.ts — auto-detect VS Code vs browser, WebSocket bridge
  with auto-reconnect and state resync on reconnect
- BottomToolbar.tsx — show hint text instead of + Agent button in CLI mode
- SettingsModal.tsx — browser file picker for layout import/export in CLI mode

Backend changes:
- src/types.ts — terminalRef now optional (CLI agents have no terminal)
- src/agentManager.ts — skip CLI agents in persistAgents
- src/PixelAgentsViewProvider.ts — null-guard all terminalRef usages

Build:
- esbuild.js — new --cli flag builds dist/cli.js with vscode stub alias
- package.json — add bin, start, build:cli scripts; ws dependency

Usage:
  npm run build:cli
  npm start  # or: node dist/cli.js --port 7842