docs(openspec): add reporting API contract documentation

Add comprehensive API documentation for the reporting endpoint:
- Request/response structure
- View type inference (did/is/will)
- Blank vs explicit zero semantics
- Status values and error responses

Related to enhanced-allocation change.

.ralph/ralph-history.json

@@ -152,14 +152,27 @@
       "errors": [
         "\u001b[91m\u001b[1mError: \u001b[0mSession not found"
       ]
+    },
+    {
+      "iteration": 10,
+      "startedAt": "2026-02-26T03:41:24.806Z",
+      "endedAt": "2026-02-26T03:43:03.164Z",
+      "durationMs": 91537,
+      "agent": "opencode",
+      "model": "",
+      "toolsUsed": {},
+      "filesModified": [
+        "openspec/changes/enhanced-allocation/"
+      ],
+      "exitCode": 1,
+      "completionDetected": false,
+      "errors": []
     }
   ],
-  "totalDurationMs": 11830,
+  "totalDurationMs": 103367,
   "struggleIndicators": {
-    "repeatedErrors": {
-      "\u001b[91m\u001b[1mError: \u001b[0mSession not found": 10
-    },
-    "noProgressIterations": 9,
-    "shortIterations": 10
+    "repeatedErrors": {},
+    "noProgressIterations": 0,
+    "shortIterations": 0
   }
 }