XY-870: Migrate Decodex review config to review levels

[yvette-carlisle]

Summary:

• Replaced Decodex review config with [codex].review levels: off, basic, standard, strict.
• Wired prompt, tracker checkpoint, and retained GitHub Review behavior to the new level semantics.
• Updated specs, example config, tests, and added the review config migration runbook.

Validation:

• cargo make fmt
• cargo make lint-fix
• cargo test -p decodex review --all-features
• cargo test -p decodex config --all-features
• cargo test -p decodex run_and_prompting --all-features (0 matched in current harness)
• cargo test -p decodex prompt --all-features
• cargo make test