Auto-import project macros from macros/ directory

[jlowin]

Add support for auto-importing Jinja2 macros from a macros/ directory (similar to dbt's approach).

Proposed behavior

Any .md file in macros/ directory gets loaded and its macros are available globally in all templates without explicit import.

project/
  macros/
    auto_tags.md       # {% macro auto_tags() %}...{% endmacro %}
    auto_title.md      # {% macro auto_title() %}...{% endmacro %}
  models/
    document.md        # Can use {{ auto_tags() }} directly

Implementation

In bind_context_to_environment(), discover and load macros from macros/*.md files.

Benefits

• Matches dbt convention (familiar pattern)
• Zero boilerplate for reusable patterns
• Project-scoped (not global)
• Enables macro libraries for common defer block patterns

Related

Follow-on from defer blocks work (#68). Common patterns like auto-tagging, TOC generation, and metadata extraction are perfect candidates for reusable macros.