feat(plane): manifest schemas and validation (task 1)

Add openultrasast.plane.manifests: frozen dataclasses and loaders for ax's
Task, Workspace and Model kinds (ax.io/v1alpha1, ax's fields only) and the
Run kind (openultrasast.io/v1alpha1: tasks, dependsOn, inputs, outputs,
budget, serialize). Every violation raises ManifestError naming the kind,
metadata.name and field path: unknown fields at any depth, wrong apiVersion
or kind, a provider outside google/anthropic unless deepseek is declared via
the openultrasast.io/provider-extension annotation, a Run with a dependency
cycle, an input naming no task, and a Run task naming no loaded Task.
load_manifests reads multi-document YAML files and indexes by kind and name.

PyYAML becomes the one declared core dependency; the two zero-dependency
guards now pin exactly that, and the module audit registers the plane
package under the ai-service-plane spec.

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01EmM1j46qgKMh3zqD1QMwEo

benchmarks/measurements/2026-09-08-module-audit.json

@@ -1,7 +1,7 @@
 {
-  "modules": 118,
+  "modules": 120,
   "counts": {
-    "load_bearing": 109,
+    "load_bearing": 111,
     "standalone_capability": 9
   },
   "orphaned": [],
@@ -645,6 +645,18 @@
         "pair_gate"
       ]
     },
+    {
+      "module": "plane",
+      "classification": "load_bearing",
+      "reason": "ai-service-plane Req 1: the model-level service plane package; reconciler and runner land in tasks 3-4",
+      "importers": []
+    },
+    {
+      "module": "plane.manifests",
+      "classification": "load_bearing",
+      "reason": "ai-service-plane Req 1.1-1.4: ax-shaped Task/Workspace/Model and the Run kind, validated before anything runs",
+      "importers": []
+    },
     {
       "module": "policy",
       "classification": "load_bearing",