[bug] Ctrl+C kills the entire session instead of canceling the current operation

## Problem

Pressing bare Ctrl+C sends SIGINT and terminates the entire altimate-code process, killing the session. Users instinctively press Ctrl+C to cancel a long-running tool call or abort a response — and lose their entire conversation, context, and unsaved state.

## Expected behavior

- **Ctrl+C** should cancel the current in-flight operation (tool call, streaming response) and return to the input prompt
- **Ctrl+D** or `/exit` should quit the session

This is standard behavior in virtually every interactive terminal application (Python REPL, node REPL, psql, etc.).

## Current behavior

- **Ctrl+C** kills the entire process (SIGINT, no graceful handling)
- **Leader + Ctrl+C** is the documented "Interrupt session" keybind (see [keybinds docs](https://docs.altimate.sh/configure/keybinds/))

The documented interrupt keybind is a two-chord sequence that most users will not discover. Bare Ctrl+C should be the interrupt, not the kill signal.

## Impact

- Users lose conversation history and context mid-session
- Especially painful during long multi-step workflows
- Muscle memory from every other terminal tool causes accidental session termination