feat(staging): per-line selection + Cmd+S shortcut

[poolcamacho]

Summary

Line-level interactive staging. The checkbox on each @@ header still stages the whole hunk, but now every individual + / - line has its own checkbox too — pick exactly which additions and deletions go into the index. Cmd+S stages (or unstages, depending on the view) the current selection without leaving the diff.

What changed

Stage 4 — per-line selection (d2a0576)

• AppState.selectedHunks: Set<Int> becomes selectedLines: [Int: Set<Int>], keyed by hunk index with values pointing at line indices inside that hunk's lines array. A hunk with every modifiable line selected collapses to the same patch the hunk-only path produced.
• DiffFile.patchText(forLines:) walks each selected hunk and rewrites the body:
  • Unselected + lines are dropped — they don't exist in the old side and we don't want them in the new side either.
  • Unselected - lines are demoted to context so the line survives the partial patch.
  • Hunk header oldCount / newCount are recomputed from the rewritten body. oldStart / newStart stay anchored.
  • Helper rewriteHunk + emit split out so cyclomatic complexity stays under the linter limit.
• GitCoordinator.stageSelectedLines / unstageSelectedLines replace the previous hunk-only methods and pipe the rewritten patch to git apply --cached (with --reverse when unstaging).
• DiffView: every + / - DiffLineView gains an optional isSelected binding with a checkbox. Context rows reserve the slot so columns stay aligned. The @@ header checkbox still toggles the whole hunk by writing all-or-nothing into selectedLines.
• HunkStagingToolbar: Select all now targets every modifiable line across all hunks; the counter reports X of Y lines.

Cmd+S shortcut + docs (0032212)

• .keyboardShortcut("s", modifiers: .command) on both Stage selected and Unstage selected buttons in the toolbar. The shortcut fires whichever action is active given the selected file's staged state.
• README: interactive staging line-level moved to Done, removed from Next, and the pending keyboard-shortcuts bullet trimmed to only mention what's still missing.

Engagement badges (5b5134d)

• Adds shields.io/github/stars|forks|issues|last-commit badges alongside the existing CI / Platform / Swift / License / Sponsors row.

Not in this PR

• Discard selected lines (the git checkout -p equivalent). Destructive; deserves its own review.
• Cmd+Enter commit and command palette — separate follow-up.

Test plan

• Repo with a file containing two modified regions → Changes tab → pick ONE + line in one hunk → Cmd+S → git diff --cached shows only that line; git diff still shows the rest
• In a hunk that mixes - and +, select only the + → stage → the - does NOT land in the index (demoted to context) and the file patches cleanly
• @@ header checkbox still selects every modifiable line in that hunk; toolbar Select all selects every line across all hunks
• Staged file view → pick lines → Cmd+S → those lines leave the index, rest stays staged
• Untracked file → no toolbar, no checkboxes
• Conflicted file → still shows ConflictView, no staging toolbar
• History tab commit diff → read-only, no checkboxes
• xcodebuild clean, swiftlint --strict clean