feat(forge): add minimal CSV exporter

[MarcoPorcellato]

Context

CONTRIBUTING cites "a new exporter in forge.py" as an example feature. A minimal CSV exporter is a bounded way to learn the FORGE visitor pattern.

Objectives

• Add CSV export visitor (or static method on ForgeExporter)
• Columns: at minimum uuid, clean_text, indent_level, parent_id
• Tests in tests/test_forge.py
• Optional CLI --format csv (discuss in PR if scope grows)
• CHANGELOG.md if user-visible

Files

• src/logseq_matryca_parser/forge.py
• tests/test_forge.py

Getting started

1. Copy pattern from JSON exporter in forge.py
2. uv sync --all-extras && make all

Out of scope

• logos_core.py model changes

Definition of Done

• make all passes