Reverse search (Ctrl+R)

[cdprice02]

Implement incremental reverse search through command history via Ctrl+R.

Behavior:

• Typing after Ctrl+R searches backwards for commands containing the search string
• Ctrl+R again jumps to the next (older) match
• Enter executes the found command
• Escape or Ctrl+C cancels the search

Acceptance criteria:

• Reverse search finds matching commands
• Repeated Ctrl+R cycles through matches
• Enter executes, Escape cancels
• Search prompt provides clear visual feedback