docs: overhaul README with AI-first structure and 8 generated demo examples

[Ne9roni]

Summary

Restructures the README to lead with AI use cases and backs it up with real generated output.

Key changes

Structure reordering:

• "AI Agent Integration" moved immediately after "Why piskel-cli?" — the AI workflow is now the second thing a reader sees, not buried near the bottom
• New "AI-Generated Demo" section replaces the old "Demo" section, with an explicit callout that no image model was used

AI Agent Integration section (now #2):

• Clear headline: "You don't need an image generation model"
• Shows the exact npx skills add install command
• Plain-language example: describe what you want → agent writes plan.json → piskel-cli run → done

AI-Generated Demo section:

• 4 PNG sprites in a side-by-side table: Mushroom, Gem, Sword, Heart
• 4 GIF animations in a side-by-side table: Coin Spin, Fire, Ghost, Twinkle Star
• Each entry links directly to its plan file so readers can inspect or run it themselves

New example plan files (6 added):

• examples/mushroom-plan.json — 16×16 red mushroom with white spots
• examples/gem-plan.json — 16×16 purple faceted gem
• examples/sword-plan.json — 16×16 diagonal sword with golden guard
• examples/coin-spin-plan.json — 4-frame coin rotation animation
• examples/fire-plan.json — 3-frame flickering fire animation
• examples/ghost-plan.json — 4-frame floating ghost animation

All output files committed to examples-output/ so the demo images render directly on GitHub.