Add available vs installed view to marr standard list

[virtualian]

Summary

marr standard list currently shows only installed standards (in the current project's .claude/marr/standards/). It would be useful to also show what standards are available from the marr package.

Proposed Behavior

# Current behavior - list installed standards
marr standard list

# New: show available standards from marr package
marr standard list --available

# New: show comparison (installed vs available)
marr standard list --all

Output Example

MARR Standards

Installed:
  ✓ prj-documentation-standard.md
  ✓ prj-testing-standard.md
  ✓ prj-workflow-standard.md

Available (not installed):
  ○ prj-ui-ux-standard.md
  ○ prj-mcp-usage-standard.md

Implementation Notes

• Available standards live in the marr package at .claude/marr/standards/
• Need to resolve package location (could use import.meta.url or similar)
• Compare filenames between package and project directories