- Add format_envelope_markdown() function in invariants.rs for rich markdown
formatting of ActionEnvelope facts
- Add format_yaml_value_markdown() helper for recursive YAML value display
- Update execute_plan_read() to append rulespec and envelope sections
- Update execute_plan_write() to append envelope section alongside rulespec
- Add 3 tests for format_envelope_markdown (empty, with facts, null values)
When plan_read or plan_write is called, the output now includes:
- Plan YAML (as before)
- Rulespec section (if rulespec.yaml exists) with invariants grouped by source
- Envelope section (if envelope.yaml exists) with facts in readable format
Missing files show placeholder text rather than errors.
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