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.

jobdefs/part-export-step.yaml

@@ -0,0 +1,24 @@
+job:
+  name: part-export-step
+  version: 1
+  description: "Export a part to STEP format"
+
+  trigger:
+    type: manual
+
+  scope:
+    type: item
+
+  compute:
+    type: export
+    command: create-export
+    args:
+      format: step
+      output_key_template: "exports/{part_number}_rev{revision}.step"
+
+  runner:
+    tags: [create]
+
+  timeout: 300
+  max_retries: 1
+  priority: 100