docs: document CLI providers, configuration, doctor, and operations

[EmersonBraun]

Parent: #37
Depends on: #38, #39, #40, #43, #44, #45, #46

Objective

Make the feature usable by a new contributor and operable in CI without tribal knowledge.

Scope

• Document grok-cli and opencode-cli IDs, support levels, prerequisites, and the ACP/headless transport rules.
• Document .agentskit-review.json version 1, precedence, enabled/required lenses, context modes, trust modes, budgets, retries, and allowIncomplete.
• Document doctor, --live, --dry-run/plan output, exit codes, incomplete states, and fork behavior.
• Add one minimal local example and one least-privilege GitHub Actions example.
• State clearly that secrets do not belong in project configuration and that repository conventions are untrusted input.
• Keep the documentation aligned with the ADR and provider matrix; update README/help text only where the user can discover the behavior.

Acceptance criteria

• A new contributor can configure and run an offline review without asking for undocumented setup.
• A maintainer can diagnose binary/version/auth/transport problems using doctor.
• CI behavior for forks, missing credentials, incomplete reviews, and no-fail is unambiguous.
• Documentation gates and README checks pass.