feat: multi-select in References preview

[gavin-jeong]

Summary

• Adds URL-menu-style multi-selection to the References preview (the PR/Jira panel opened via refs/8):
  • Space toggles the ref under the cursor into a selection set and advances the cursor.
  • Enter/o opens every selected ref's URL in the browser, falling back to the ref under the cursor when nothing is selected.
  • y copies the selected ref URL(s) to the clipboard, newline-joined.
• Selection state is cleared when the session changes, when leaving refs mode (esc, tab-cycle, or re-toggling the mode key), and is folded into the render cache key so a toggle actually repaints the pane.
• Title hint and bottom-of-list N selected count added, matching the existing URL menu's UX.

Bug found while manually driving the app

toggleRefSelection originally only cleared sessRefsCacheKey without calling updateSessionRefsPreview to actually repaint sessSplit.Preview's content. Unit tests passed because they call the render function directly, but running the built binary in tmux showed the selection mark never appeared on screen even though the underlying state was correct (confirmed via y's "Copied N reference(s)" message and clipboard contents). Fixed by having toggleRefSelection return the tea.Cmd from updateSessionRefsPreview, same pattern already used by cursor-move handling.

Test plan

• go build ./..., go vet ./..., go test ./... all pass.
• Added tests: space toggles selection, enter opens all selected (not just cursor), y copies selected (falls back to cursor item), selection clears when leaving refs preview.
• Manually drove the built binary in tmux: verified selection marks render, "N selected" counter updates, y copies the right URLs (checked via pbpaste), and selection clears on esc + re-entry.