feat: add --skip-checks to exclude specific checks

[mvvmm]

Adds skipCheckIds as an exclude-list counterpart to the existing checkIds include-list. Useful when you want to disable one or two checks without enumerating all the others.

Behaviour:

• Skipped checks emit status: "skip" without running
• Excluded from scoring (matching existing skip handling)
• Does not cascade-skip dependent checks — dependents run in standalone mode, consistent with how checkIds filtering works

Supported via:

• CLI: --skip-checks <ids>
• Config file: skipChecks: [...]
• Programmatic API: skipCheckIds in RunnerOptions

# Disable a single check
afdocs check https://docs.example.com --skip-checks markdown-content-parity

# agent-docs.config.yml
skipChecks:
  - markdown-content-parity

const report = await runChecks('https://docs.example.com', {
  skipCheckIds: ['markdown-content-parity'],
});

[dacharyc]

Thank you for this, @mvvmm - it's a nice ergonomic improvement! I'm going to merge it as-is and make a couple of small follow ups - I'm going to add validation when --checks and --skip-checks overlap, and update the programmatic API doc example so it doesn't show both being used simultaneously, which is potentially confusing.

Really appreciate this batch of PRs! 🎉