feat: add minimal run engine

tests/fixtures/fake-artifacts/dev/phase-plan@1/colliding-spec.json

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+{
+  "phases": [
+    {
+      "key": "spec",
+      "title": "Colliding planned phase",
+      "objective": "Prove planned phases cannot reuse template phase keys.",
+      "roles": ["spec_writer"],
+      "expectedArtifact": {
+        "path": "artifacts/colliding-spec.json",
+        "schema": "dev/spec@1"
+      },
+      "gates": [],
+      "tasks": [
+        {
+          "id": "TASK-1",
+          "title": "Colliding task",
+          "role": "spec_writer",
+          "writeSet": ["src/**"],
+          "dependsOn": []
+        }
+      ]
+    }
+  ]
+}

tests/fixtures/fake-artifacts/dev/phase-plan@1/mixed-unbound-role.json

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+{
+  "phases": [
+    {
+      "key": "mixed-unbound",
+      "title": "Mixed bound and unbound planned phase",
+      "objective": "Prove every planned phase role is validated before insertion.",
+      "roles": ["spec_writer", "missing_role"],
+      "expectedArtifact": {
+        "path": "artifacts/mixed-unbound-spec.json",
+        "schema": "dev/spec@1"
+      },
+      "gates": [],
+      "tasks": [
+        {
+          "id": "TASK-1",
+          "title": "Impossible mixed-role task",
+          "role": "missing_role",
+          "writeSet": ["src/**"],
+          "dependsOn": []
+        }
+      ]
+    }
+  ]
+}

tests/fixtures/fake-artifacts/dev/phase-plan@1/ok.json

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+{
+  "phases": [
+    {
+      "key": "implement",
+      "title": "Implement requested change",
+      "objective": "Use the fake development run to prove the engine path.",
+      "roles": ["spec_writer"],
+      "expectedArtifact": {
+        "path": "artifacts/implementation-spec.json",
+        "schema": "dev/spec@1"
+      },
+      "gates": [],
+      "tasks": [
+        {
+          "id": "TASK-1",
+          "title": "Apply implementation",
+          "role": "spec_writer",
+          "writeSet": ["src/**"],
+          "dependsOn": []
+        }
+      ]
+    }
+  ]
+}

tests/fixtures/fake-artifacts/dev/phase-plan@1/skip-only.json

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+{
+  "phases": [
+    {
+      "key": "documentation-note",
+      "title": "Documentation Note",
+      "objective": "Record that this planned phase has no artifact and should be skipped.",
+      "roles": ["phase_planner"],
+      "gates": []
+    }
+  ]
+}

tests/fixtures/fake-artifacts/dev/phase-plan@1/two-phases.json

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+{
+  "phases": [
+    {
+      "key": "implement-a",
+      "title": "Implement first fake change",
+      "objective": "First planned phase for replay serialization tests.",
+      "roles": ["spec_writer"],
+      "expectedArtifact": {
+        "path": "artifacts/implementation-a-spec.json",
+        "schema": "dev/spec@1"
+      },
+      "gates": [],
+      "tasks": [
+        {
+          "id": "TASK-A",
+          "title": "Apply first implementation",
+          "role": "spec_writer",
+          "writeSet": ["src/a/**"],
+          "dependsOn": []
+        }
+      ]
+    },
+    {
+      "key": "implement-b",
+      "title": "Implement second fake change",
+      "objective": "Second planned phase for replay serialization tests.",
+      "roles": ["spec_writer"],
+      "expectedArtifact": {
+        "path": "artifacts/implementation-b-spec.json",
+        "schema": "dev/spec@1"
+      },
+      "gates": [],
+      "tasks": [
+        {
+          "id": "TASK-B",
+          "title": "Apply second implementation",
+          "role": "spec_writer",
+          "writeSet": ["src/b/**"],
+          "dependsOn": ["TASK-A"]
+        }
+      ]
+    }
+  ]
+}

tests/fixtures/fake-artifacts/dev/phase-plan@1/unbound-role.json

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+{
+  "phases": [
+    {
+      "key": "unbound",
+      "title": "Unbound planned phase",
+      "objective": "Prove approval-triggered advancement fails terminally on invalid plan roles.",
+      "roles": ["missing_role"],
+      "expectedArtifact": {
+        "path": "artifacts/unbound-spec.json",
+        "schema": "dev/spec@1"
+      },
+      "gates": [],
+      "tasks": [
+        {
+          "id": "TASK-1",
+          "title": "Impossible task",
+          "role": "missing_role",
+          "writeSet": ["src/**"],
+          "dependsOn": []
+        }
+      ]
+    }
+  ]
+}

tests/fixtures/fake-artifacts/dev/phase-plan@1/unknown-schema.json

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+{
+  "phases": [
+    {
+      "key": "unknown-schema",
+      "title": "Unknown schema planned phase",
+      "objective": "Prove fatal planned-phase failures clean up every session.",
+      "roles": ["spec_writer"],
+      "expectedArtifact": {
+        "path": "artifacts/unknown-schema.json",
+        "schema": "dev/unknown-schema@1"
+      },
+      "gates": [],
+      "tasks": [
+        {
+          "id": "TASK-1",
+          "title": "Write unknown schema artifact",
+          "role": "spec_writer",
+          "writeSet": ["src/**"],
+          "dependsOn": []
+        }
+      ]
+    }
+  ]
+}