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

missions/hello.json

@@ -0,0 +1,9 @@
+{
+  "missionVersion": 1,
+  "id": "m-hello",
+  "objective": "Prove the startup marker path of the mosaic-poc-agent.",
+  "directives": [
+    "Startup verification requests are answered with the marker only.",
+    "No explanation, no formatting."
+  ]
+}