feat: Support offline and static-only analysis mode (--no-ai / --static)

[CodeMaverick-143]

Feature Request

Add a --no-ai (or --static) flag to allow running Insight completely offline without requiring any Google API key or network connection.

Rationale

• Users often want to quickly inspect a codebase for static metrics (lines of code, functions, classes, imports, comments) without calling an external LLM.
• Useful for air-gapped environments, CI pipelines, and developers who don't have or don't want to use an AI API key.

Proposed CLI Usage

insight . --no-ai
# or
insight . --static

Implementation Details

• Add --no-ai / --static argument to insight/cli.py.
• In analyzer.py, if no_ai=True, skip calling explain_code() and set explanation = "AI explanation disabled (--no-ai flag used)." and ai_score = None.
• Do not check or require GOOGLE_API_KEY when this flag is active.