feat: pull-request actions, floating diffs, home-view polish

[777lotto]

• Pull Requests view rows gain go/gd/gc/gm: check out, diff vs base, comment, and merge (confirm-gated; deletes the branch remote+local, returns to base and fast-forwards when you were on the PR branch)
• github.lua: Client:mutate write path (merge_pull/comment_pull/ delete_branch) through the same gh->curl transport chain; fail-closed guards; empty 204 bodies decode to nil
• show_scratch opens a centered rounded float (q/ close) instead of a bottom split - commits, pushes, GitHub details, PR diffs all float
• home view: empty sections render as one dimmed header line, noise placeholders dropped, hint-free dividers, glyph tab bar, branch trackshort rendered as arrows/check
• tests: mutation coverage added; smoke tab assertion updated

What changed

Scope

• Area:
• Platform:

Validation

• I targeted bluff, or this is a bluff → bet promotion.
• Commits are signed and show as verified on GitHub.
• Lua compilation, focused unit tests, and the Git fixture smoke test pass.
• :help git-panel matches user-visible behavior.
• I updated CHANGELOG.md for a user-visible change.
• I added a screenshot or recording for a visible UI change.