Specify faster collapsible task navigation

[isomorphisms]

Stacks on #24, which establishes the ChatGPT-like text task frontend.

This records the left-navigation interaction before a frontend toolkit is chosen:

• collapsible left navigation without disturbing the active task;
• literal ASCII > when closed to open rightward;
• literal ASCII < when open to close leftward;
• no hamburger toggle;
• approximately 150 ms normal-motion transition target;
• immediate reduced-motion path;
• rapid toggles converge on the latest requested state rather than queueing stale animation;
• navigation visibility remains frontend presentation state and does not mutate browser-owned tasks/tabs/history.

This is deliberately an interaction/acceptance contract, not a premature UI-framework implementation.