The parser hardcoded 4/6/8/10-space indent levels for must_haves
sub-blocks, but standard YAML uses 2-space indentation. This caused
"No must_haves.key_links found in frontmatter" for valid plan files.
The fix dynamically detects the actual indent of must_haves: and its
sub-blocks instead of assuming fixed column positions.
Fixes#1356
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
- spliceFrontmatter: replace existing, add to plain content, exact body preservation
- parseMustHavesBlock: truths as strings, artifacts as objects with min_lines,
key_links with from/to/via/pattern, nested arrays, missing block, no frontmatter
- FRONTMATTER_SCHEMAS: plan/summary/verification required fields, all schema names
