docs: reshoot the screenshots for v0.7.0

[handlename]

Why this change is necessary

All four figures in the repository were taken at v0.6.0. #67 replaced the comment indicator's colour emoji with a drawn glyph, and every figure still shows the old chip — so each of them now contradicts UI_DESIGN section 2.1 in exactly the place that PR existed to fix. A release is the moment a reader arrives at the README, so the pictures have to be the ones they will actually see.

Approach

All four were reshot at the conventional dimensions and theme: light, 1440x900 @2x for the README pair, 3984x2340 for the anatomy figures, taken from real MCP review rounds against examples/ rather than from hand-written sidecars.

The Markdown README shot also changes composition. Two threads now hang off one requirement — one awaiting an answer, one resolved and folded away — so the release's headline change (#70, several threads per block) and the folded resolved card (#63) are both in the first picture a reader meets. Its alt text follows the picture.

The diff shot and the two anatomy figures keep their compositions: what they demonstrate did not change, and holding them still keeps the diff to the one thing that did. GLOSSARY section 5 already names the folded card's identifiers, so no term moved and no label had to be added to either figure.

Review Points

• Whether the Markdown shot is worth the busier composition. The alternative was to keep the single-thread frame and change only the indicator, which would have been a smaller diff but would have left v0.7.0's headline change invisible in the README.
• The anatomy figures are regenerated from measured bounding boxes rather than hand-placed, so a label that looks off is a layout question rather than a positioning slip. Their terms are the ones GLOSSARY section 5 defines.