Fix/270 claude install

[kavish-100403]

Summary

Fixes darnit install writing MCP config to the wrong file for Claude Code.

• Default client is now claude-code (was ambiguous claude)
• Claude Code (global): writes ~/.claude.json (was ~/.claude/settings.json, which is Claude Desktop)
• Claude Code (--project): also writes .mcp.json at the repo root (MCP was global-only before)
• Claude Desktop: --client claude-desktop keeps ~/.claude/settings.json
• Deprecation: --client claude still works but logs a warning; use claude-code or claude-desktop

Type of Change

• Bug fix (non-breaking change fixing an issue)
• New feature (non-breaking change adding functionality)
• Breaking change (fix or feature causing existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

Framework Changes Checklist

If this PR modifies the darnit framework (packages/darnit/):

• Updated framework spec (docs/architecture/framework-design.md) if behavior changed
• Ran uv run python scripts/validate_sync.py --verbose and it passes

Control/TOML Changes Checklist

If this PR modifies controls or TOML configuration:

• Control metadata defined in TOML (not Python code)
• SARIF fields (description, severity, help_url) included where appropriate
• Ran validation to confirm TOML schema compliance

Testing

• Tests pass locally (uv run pytest tests/ -v)
• Added tests for new functionality (if applicable)
• Linting passes (uv run ruff check .)

Additional Notes

Files changed:

• packages/darnit/src/darnit/cli.py — path logic, argparse choices, deprecation warning
• tests/darnit/test_cli.py — tests for claude-code, claude-desktop, --project, deprecation
• docs/getting-started/using-skills.md — install commands and config file table
• docs/install/from-source.md — darnit install is now the recommended path for Claude Code