[omartelo]

Summary

• Overview › Description: the Edit button beside the heading becomes a ⋯ menu, as GitHub's comments have: Copy link (the pull request's URL), Copy Markdown (the raw body, disabled when empty), a separator, and Edit, which opens the field as the button did. Two new strings land in every locale.
• Task lists: goldmark's TaskList extension (already a dependency) is on; a [x]/[ ] item draws the UI kit's Checkbox, read-only, in the bullet's place, centred on the first line. The marker column widens to the box.
• Heading rules: headings are blocks of their own; h1 and h2 draw th.Separator under them, and every heading uses GitHub's 1.25 line height so the rule sits close to its text, with the usual block gap below.
• CHANGELOG.md has the entries.

Tests: TestPullsDescriptionCancelInTheFrameThatTookTheClick now opens the ⋯ before clicking Edit (the contract changed: Edit moved into the menu); its assertion is unchanged. The Markdown test theme gets a surface (th.On(th.Background)), which the separator and checkbox need to draw, as every real window has.

Test plan

• New tests: task items draw a box (checked, unchecked, plain item) without a leftover [x], and the marker column fits the box; only h1/h2 carry a rule.
• native/: gofmt, vet and tests green, after rebasing on main.
• Rig window on feat(native): show author avatars in the Pulls conversation #1002: Test plan checkboxes aligned with the first line, ⋯ menu entries, Summary/Test plan rules close to the heading.