fix: cancel current command/stream on Ctrl+C instead of exiting

[aryansk]

Problem

Pressing Ctrl+C while the assistant is streaming a reply, or while a bash
command is running, terminates the entire GCode session and prints the
goodbye screen. Any messages since the last save are lost — the turn in
progress is never persisted. Closes #51.

Change

Three interrupt-handling fixes:

• execute_bash (gcode/tools.py): catches KeyboardInterrupt from
  subprocess.run and returns Command execution cancelled by user.
  instead of letting it kill the REPL. Applies both to the prompt phase
  (already handled) and the running-command phase (new).
• Streaming (gcode/agent.py _stream): a Ctrl+C during streaming
  stops the stream, keeps whatever was accumulated so far as the turn's
  assistant message (with no half-formed tool calls), and persists it —
  the session stays alive and history is saved.
• Tool calls (gcode/agent.py _run_tool): a Ctrl+C during any tool
  call returns Command execution cancelled by user. to the model instead
  of propagating.

A Ctrl+C (or Ctrl+D) at the prompt still exits with the goodbye screen,
unchanged.

Why this approach

Minimal, targeted changes: the interrupt is caught at the two async
boundaries where it can arrive mid-work (streaming and subprocess), and the
tool-call boundary is hardened so a cancelled tool result is recorded in
history like any other tool result. No changes to the prompt loop or exit
paths.

Testing

uv run pytest:  111 passed (4 new: 3 agent streaming/tool-cancel tests,
                1 subprocess-interrupt test)
uv run ruff check .:  All checks passed
uv run ruff format --check .:  31 files already formatted
uv run mypy gcode:  Success: no issues found in 12 source files
uv run bandit -q -r gcode/ -c pyproject.toml:  clean
uv run python -m compileall -q gcode demo:  clean

New tests cover: partial reply retained on mid-stream interrupt, empty
reply when interrupted before any chunk, and a tool that raises
KeyboardInterrupt returning the cancelled result.

Documentation and release impact

• User-facing documentation updated
• Changelog/release note needed (user-visible behavior change)
• Migration or compatibility note needed
• No documentation impact

Review notes

• Known limitations: a Ctrl+C during streaming intentionally drops any
  tool calls the model had begun to emit (they are half-formed), but keeps
  the text.
• Follow-up issue: none.