feat: VS Code status line companion + statusline usage limits

Add a VS Code companion extension (vscode-extension/) that surfaces Token
Optimizer's context health, efficiency, warnings, compactions, session duration,
active agents, and authoritative 5h/7d usage limits in the editor status bar
with a rich hover tooltip and click-to-dashboard. Works in VS Code, Cursor, and
Windsurf via a cache-file bridge that reads the data the hook and status line
already write, with an opt-in, off-by-default OAuth path for always-fresh usage
limits when no terminal is open. Zero runtime dependencies.

Extend statusline.js to capture authoritative rate_limits to a global sidecar
(account-global, atomic write) and render a compact 5h/7d row in the terminal
status line.