docs: update for v0.12.0

[schovi]

README updates for release v0.12.0, advertising the context frames feature that shipped in the release.

• Added a feature-table row describing the context frames below the worktree table (PR review roll-up, per-file changes summary, Git context graph).
• Expanded the GitHub Integration section to describe the lazy-loaded PR review frame: state/checks/reviews roll-up, failing/running checks, change-request previews, and inline review threads as terminal hyperlinks.

The README badge for the latest version is dynamic, so no version string needed editing. The v0.12.0 CHANGELOG entry already landed on main with the release commit.