Refresh the demo GIF for the 0.2.0 flow

[abhiksark]

Part of the 0.3.0 adoption roadmap — Discovery.

Why (adoption impact)

The demo GIF is the highest-converting element for a TUI — people need to see the edit→save→advance loop. It should reflect the current (0.2.0) experience and any first-run improvements from this milestone.

Current state

• A GIF exists at docs/assets/demos/pylings-demo.gif, generated from docs/demo.tape via VHS. See docs/DEMO_GIF.md.

Scope / acceptance criteria

• Regenerate the GIF with VHS so it shows the current flow (and the new no-arg launch from Submit Pythonlings to eligible CLI/TUI directories #9 if that landed first).
• Keep it short (<30s), readable font size, shows: launch → fix an exercise → auto-advance → hint → progress.
• Confirm it renders in the README Demo section and isn't oversized (keep file weight reasonable).

Where to look

• docs/demo.tape, docs/DEMO_GIF.md, docs/assets/demos/pylings-demo.gif.

Difficulty

good first issue — requires installing VHS; ~45 min.

[abhiksark]

Closing as shipped: the current demo refresh landed in #30 (following #29) and is reflected in the repository demo documentation.