Architecture updates to support the plugin's I/O and configuration layer:
- TaskSource interface abstracts task loading from I/O, making future
sources (API, database, test) swappable without operation changes
- FileSource implements v1: Bun.Glob for directory scanning, Bun.file
for reading, parseFrontmatter for parsing (single-pass I/O)
- SourceResult provides raw file content (for show) and per-file error
detail (for validate) that parseTaskDirectory couldn't offer
- Config schema uses TypeBox (already a dep via taskgraph) for
compile-time types, runtime validation, and JSON Schema export
- ADR-005: TaskSource abstraction rationale
- ADR-006: Bun.Glob over parseTaskDirectory rationale
- Performance benchmark added (43 tasks full pipeline: ~150ms)
- AGENTS.md updated with config section and source structure
