docs: Phase 1 handoff — Rust CLI canonical, Python frozen; v0.3.0 drops the console script (LEO-342)

The documentation handoff closing Phase 1: the Rust CLI is the canonical
sensorwatch interface and the Python package is frozen as the reference
implementation.

- Drop the [project.scripts] console-script entry point; bump the Python
  package to 0.3.0 (pyproject + __init__ + uv.lock) and roll the CHANGELOG
  Unreleased section into [0.3.0]. The logger stays runnable as
  python -m sensorwatch; installs of <=0.2.0 keep the old script until
  upgraded.
- Retire the usage skill's bundled scripts/snapshot.py helper (superseded by
  the Rust snapshot subcommand) and update every reference (SKILL.md,
  README, SECURITY.md, ROADMAP, snapshot.rs doc comment, CLI README).
- README: canonical-CLI framing, an explicit bare-invocation -> log migration
  note, the frozen-reference-implementation statement, Windows-native command
  forms for the live-read examples (LEO-413 residual), and a rewritten
  install collision note.
- ROADMAP: close the Binary naming open question as Decided (LEO-342);
  Phase-1-close charter marked shipped; status rows updated.
- CONTRIBUTING: getting-started builds the Rust CLI first; version-streams
  table bumped; bug-report asks for the toolchain actually in use.
- Monitor skill: label the LEO-340 supervisor / dead-man's-switch wiring as
  external operator-side (public equivalent planned in LEO-458).
- examples/monitor-setup + WSL2 doc: collision caveats reworded for the
  post-drop world; console-script launch form removed.

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