feat(image): add Image Color Picker

[hateofhades]

Closes #121.

The last planned Gizlet of roadmap phase 2. The registry entry flips to available with an agent block, it declares itself flowless, gains a related-Gizlet key, a social card and page copy, and its bench row goes.

What it does

Pick a pixel and read it as HEX, RGB and HSL, each copyable. The picture is drawn onto a canvas in this browser and each pick reads that one pixel back — nothing uploaded, no file written.

• Pointer or touch: click, or hold and drag to keep picking as you sweep across an area.
• Keyboard: focus the picture and use the arrow keys — one pixel a step, ten with Shift, stopping at the edge. It opens on the middle pixel, so there is always a colour to move from rather than an empty panel to aim at, and the canvas's accessible name carries the current colour and coordinates as they change.
• The colours you picked sit underneath, newest first, each copyable by clicking it. A repeat moves rather than duplicating, because picking a colour twice is how somebody checks they picked it. They live in the page and nowhere else — localStorage is untouched, and closing the tab forgets them.

Two decisions in the colour maths

• A grey has no hue. Every hue produces it once saturation is gone, so it is reported as 0 rather than as whatever the arithmetic left behind — the convention browsers print too.
• The swatch chooses its own text colour by relative luminance, not lightness. Yellow is the case a lightness threshold gets wrong: bright, but a white label on it is unreadable. That is a unit test rather than a screenshot.

No flow contract, deliberately

Like Image Dimensions, it shows the visitor something and produces nothing another Gizlet could take, so it joins flowlessToolSlugs rather than being given a pass-through contract it does not have.

Checks

pnpm run check, pnpm test (500), pnpm run build and pnpm run test:e2e (119) all pass. New coverage: tests/unit/image-colour.test.ts (hex padding, the wheel's corners, greys, the negative-hue case, hex round-trips, luminance, the history rules) and tests/e2e/image-color-picker.spec.ts, which picks from a four-colour fixture and asserts the exact values, copies to a real clipboard and reads it back, walks the picture with the keyboard alone including the edge stop, and checks a repeat does not duplicate.

Note for whoever merges

One of several image Gizlets cut from main in parallel (#156, #157, #158). They touch the registry, the related-Gizlet map, flowlessToolSlugs and the regenerated gizlets.png; whichever lands last needs a rebase, and the conflicts are additions next to each other.

Screenshot