Vi mode: add 's' (substitute) command

Summary

Vi mode is missing the s command (substitute - delete character under cursor and enter insert mode). This is a commonly used vi command, essentially equivalent to cl or xi.

Current state

Vi mode has been steadily gaining commands across releases:

• Basic motions and operators (c, d, w, b, e, etc.)
• f/F/t/T with ; and , repeat
• y/yy/Y, p/P (yank/paste)
• Text objects (iw, aw, i", a", etc.)
• >>, << (indent/dedent), J (join lines)
• u (undo)

The s command is not currently supported. Neither is S (substitute entire line, equivalent to cc).

Expected behavior

• s in normal mode: delete character under cursor and enter insert mode
• S in normal mode: delete entire line contents and enter insert mode (same as cc)
• Both should support count prefixes (e.g., 3s deletes 3 characters and enters insert mode)

Why

s is muscle memory for many vim users - it's one of the most common single-char editing commands. Its absence breaks flow when editing prompts.