feat: add YAML job definition parser and example definitions
New package internal/jobdef mirrors the schema package pattern:
- Load/LoadAll/Validate for YAML job definitions
- Supports trigger types: revision_created, bom_changed, manual, schedule
- Supports scope types: item, assembly, project
- Supports compute types: validate, rebuild, diff, export, custom
- Defaults: timeout=600s, max_retries=1, priority=100
Example definitions in jobdefs/:
- assembly-validate.yaml: incremental validation on revision_created
- part-export-step.yaml: STEP export on manual trigger
11 unit tests, all passing.
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