docs: address PR #42 round-2 review — prog string, example alias, stale version pins

- __main__.py: argparse prog now "python -m sensorwatch" — --help no longer
  advertises a console script the package stopped installing.
- examples/monitor-setup (README + PILOT_TEST_PLAN): the preamble now defines
  a one-time alias for the repo-built binary, so every bare sensorwatch
  command pastes as written instead of contradicting the not-on-PATH note.
- CHANGELOG 0.3.0 / sensorwatch.hpp: the two stale Python-version strings
  (version-streams parenthetical, mirrored-binding comment) bumped to 0.3.0.
- README migration note tightened per round-1 polish: bare sensorwatch log
  applies only once the built binary is on PATH.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>