fix(skills): align review guidance with CLI 0.7

[nehal-a2z]

What changed

• Replace legacy hidden -t review selectors with the public --committed and --uncommitted options.
• Keep the default scope flag-free so it reviews tracked changes.
• Clarify that --dir may target a directory inside an initialized Git working tree.
• Remove the separate authentication preflight and let the review command start its built-in auth flow when needed.
• Apply the same contract to the canonical skill, Claude command and agent guidance, README, and unreleased changelog.

Why

CodeRabbit CLI 0.7 uses named public scope options, supports reviewing a subdirectory within a larger repository, and authenticates automatically when a review starts. Installed skills and native plugin guidance should rely on that public behavior.

This deliberately does not include the open skill-collision rename or other unrelated changes.

Validation

• git diff --check
• Verified command mapping:
  • all -> review --agent
  • committed -> review --agent --committed
  • uncommitted -> review --agent --uncommitted
• Confirmed no remaining --plain, legacy -t invocation, standalone-repository requirement, or manual auth preflight in the distributed review guidance.