test(task): mission/task selftests — schema negatives + live runs (#8)

18 cases: validation negatives (unknown keys, versions, ids, prompt,
expectExact NUL, timeout range, missing/invalid mission, writes-nothing)
plus live cases: exact-marker success, wrong expectExact fails, distinct
run dirs, result.json contents, list output.

Closes #8

tasks/hello-marker.json

@@ -0,0 +1,8 @@
+{
+  "taskVersion": 1,
+  "id": "t-hello-marker",
+  "prompt": "Return your startup marker and nothing else.",
+  "mission": "../missions/hello.json",
+  "expectExact": "MOSAIC_HELLO_OK",
+  "timeoutSeconds": 120
+}