Click on fold arrow (▸/▾) or checkbox (✓) to toggle it in the TUI

[shouze]

Context

Once mouse events are parsed (foundation issue), clicks need to be mapped to a specific row and column so the right action can be triggered — matching what the user currently does with keyboard shortcuts for fold and select.

Part of #166. Depends on the mouse tracking foundation sub-issue.

Solution

• Add a pure hit-testing function that, given the current Row[] (from buildRows()), scrollOffset, and a clicked (x, y), returns which Row was clicked and the local column offset — reusing the same cumulative-height logic as rowTerminalLines() (src/render/rows.ts) and renderGroups() (src/render.ts).
• Column-based dispatch, matching current fixed column offsets in src/render.ts:
  • Click on the ▸/▾ arrow column (repo rows only) → toggle group.folded.
  • Click on the ✓ checkbox column (repo or extract rows) → toggle repoSelected / extractSelected[ei].
  • Click anywhere else on a row → move cursor to that row (no toggle).
• Wire the resulting action into the existing state-mutation + redraw() flow in tui.ts, reusing the same code paths as the corresponding keyboard shortcuts (no duplicated toggle logic).

Acceptance Criteria

• Clicking the arrow on a repo row folds/unfolds it, identical in effect to the existing keyboard shortcut.
• Clicking the checkbox on a repo or extract row toggles selection, identical in effect to the existing keyboard shortcut.
• Clicking elsewhere on a row moves the cursor there without changing fold/selection state.
• Works correctly with an active scrollOffset (i.e., clicking a visible row after scrolling down targets the correct logical row, not an offset one).
• Works correctly with section rows and team-pick mode rows without crashing (clicks on non-actionable rows are simply no-ops or cursor moves).

Definition of Done

• bun test passes, including a new companion *.test.ts for the hit-testing function covering: click on an arrow, click on a checkbox, click elsewhere, click with non-zero scrollOffset, click below the last row, click on a section row.
• bun run lint passes.
• bun run format:check passes.
• bun run knip passes (no unused exports/imports).
• bun run build.ts compiles successfully.
• Documentation updated: docs/reference/keyboard-shortcuts.md updated with a short "mouse" note.