[FEATURE] Add missing ghostcheck check-rules CLI subcommand

[KbWen]

Problem Description

The README documents a ghostcheck check-rules command that audits AI agent instruction files (.agent/, .cursor/, .agentcortex/). However, cli.py only implements scan, check-deps, check-secrets, �aseline, list-checks, list-plugins, and honeypot subcommands. The check-rules command is missing from the actual CLI implementation.

Expected Behavior

Running ghostcheck check-rules should invoke only the agent rules linter (similar to how check-deps invokes only the hallucination checker and check-secrets invokes only secret-related scanners).

Proposed Implementation

1. Add a check-rules subparser in cli.py (following the pattern of check-deps/check-secrets).
2. Wire it to call scanner.scan_rules() which already exists in scanner.py.
3. Update the help text and verify with a smoke test.

Difficulty: Low

Single-file change following existing patterns.