feat: navigate jump history with Ctrl+o and Ctrl+i

src/main.rs

@@ -18,6 +18,7 @@ Normal: i/a insert, I/A insert at first nonblank/end, o/O new line
         h/j/k/l move, 0/$ line ends, x delete, J join lines
         w/b/e word next/previous/end, W/B/E whitespace-delimited WORDs
         gf follows Markdown file links/reference links and heading fragments (no LSP)
+        Ctrl+o older jump location; Ctrl+i/Tab newer (counts supported; Insert Tab unchanged)
         Normal/Visual: prefix motions with counts (5j, 2w, 3n); Esc cancels count
         u undo, U redo, : command, / search, n/N next/previous match
         Normal/Visual: . repeats last native edit at current targets (one undo group)