Compiler
The compiler walks a directory tree and produces a graph. Given a root directory of markdown files with YAML frontmatter, it outputs nodes (Things) and edges (hierarchy + links).
The compiler resolves leaf vs composite nodes, parses frontmatter, extracts links, builds the parent/child hierarchy, generates backlinks, and emits validation warnings for malformed content.
Companion files (design.md, SKILL.md, AGENT.md, README.md) and system directories (_ prefix) are excluded from compilation.
See src/compiler/design.md for the reference implementation spec.