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/assembly-validate.yaml

@@ -0,0 +1,26 @@
+job:
+  name: assembly-validate
+  version: 1
+  description: "Validate assembly by rebuilding its dependency subgraph"
+
+  trigger:
+    type: revision_created
+    filter:
+      item_type: assembly
+
+  scope:
+    type: assembly
+
+  compute:
+    type: validate
+    command: create-validate
+    args:
+      rebuild_mode: incremental
+      check_interference: true
+
+  runner:
+    tags: [create]
+
+  timeout: 900
+  max_retries: 2
+  priority: 50