feat: add atomic output writes and clean cancellation

[Monotox]

Summary

Route every on-disk write through a temp-file-then-os.replace helper so a killed or interrupted run never leaves a truncated artifact; the destination is either the prior content or absent. Add CliExitCode.CANCELLED (14) and a SIGTERM/SIGINT handler installed by the console-script entrypoint so an interrupt produces a defined exit code instead of the shell default.

Test Plan

• uv run pytest passes
• uv run ruff check . passes
• Documentation updated, if user-facing behavior changed
• Golden fixtures updated, if a public JSON or Markdown contract changed

Contract Impact

• No public contract surfaces changed
• Public contract changes are intentional and versioned per docs/VERSIONING_POLICY.md