docs: Campaign LA — pinned launch-contract schema COMMITTED into plan LA1

The LA3 Opus review process note was right: the contract both sides
implement lived only in orchestrator prompts, which is exactly the drift
mode the pin exists to prevent (and it produced the paths-key CRITICAL).
The schema, field rules, probe-mode discriminator, and status vocabulary
are now a binding plan section; amendments change this text first,
implementations second. Ledger: LA3 fix round dispatched.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/Fixtures/campaign-la/session-config-shared-fixture.json

@@ -0,0 +1,20 @@
+{
+  "version": 1,
+  "sessions": [
+    {
+      "id": "shared-fixture",
+      "endpoint": { "host": "127.0.0.1", "port": 9000 },
+      "account": "sharedaccount",
+      "character": { "name": "SharedToon" },
+      "policy": { "id": "idle" },
+      "credential": {
+        "provider": "environment",
+        "reference": "SHARED_FIXTURE_PASSWORD"
+      },
+      "plugins": ["ExamplePlugin", "AnotherPlugin"],
+      "loginCommands": ["/tell someone, hi", "/vt start"],
+      "loginCommandDelayMs": 750,
+      "statusFile": "shared-fixture-status.jsonl"
+    }
+  ]
+}