feat: history-search-forward and history-search-backward

[nueces]

Summary:

Add support for history-search-backward and history-search-forward via configuration file readline-search.json in Pi's agent directory (~/.pi/agent, or PI_CODING_AGENT_DIR when set).

From GNU readline docs

history-search-backward
Search backward through the history for the string of characters between the start of the current line and the point. The search string must match at the beginning of a history line. This is a non-incremental search. By default, this command is unbound, but may be bound to the Page Down key on some keyboards.

history-search-forward
Search forward through the history for the string of characters between the start of the current line and the point. The search string must match at the beginning of a history line. This is a non-incremental search. By default, this command is unbound, but may be bound to the Page Up key on some keyboards.