feat(task): fixture mission/task, docs, recorded M2 E2E (#9)

- missions/hello.json + tasks/hello-marker.json committed fixtures
- README: Missions & tasks section (usage, run record layout, M2 scope note)
- BUILD-LOG: Phase 6 before/after entries

E2E: fixture run succeeded with exactly MOSAIC_HELLO_OK; wrong expectExact
recorded status failed (expect-mismatch) and exited 1; runs listed; config
checksum unchanged.

Closes #9

BUILD-LOG.md

@@ -137,4 +137,30 @@ All 11 acceptance criteria demonstrated. The real model request passed.
 
 Configuration-driven Hello World verified. `main` merged with M1 and tagged `config-hello-v1`.
 
+---
+
+## Phase 6: Mission and task abstraction (M2)
+
+### Entry 6.1 — before
+
+- Timestamp: 2026-09-03
+- Intended action: Add the first mission/task layer, host-side only (Gitea milestone M2, issues #6-#9): strict v1 schemas for missions and tasks, a task runner executing through the proven config-driven container path, immutable write-once run records under `<dataRoot>/runs/`, `expectExact` gating, timeouts, selftests, fixtures, and docs.
+- Reason: The foundation plan's following layer — mission (objective + directives), task (bounded unit), run (one attempt), result (immutable evidence) — must exist as data and records before any policy or multi-agent work.
+- Expected result: `scripts/run-task.sh tasks/hello-marker.json` succeeds with exactly `MOSAIC_HELLO_OK`; wrong expectations fail; every run leaves an immutable record; configuration remains untouched.
+
+### Entry 6.2 — after
+
+- Timestamp: 2026-09-03
+- Commands run: `scripts/test-task.sh` (18 cases incl. live runs); `scripts/run-task.sh validate` / `run` on committed fixtures; `node scripts/mosaic-task.mjs list`; config checksum comparison across runs.
+- Observed result:
+  - Selftests: 18 passed, 0 failed (schema negatives; live exact-marker success; wrong expectExact fails; distinct run dirs; result.json contents; list).
+  - Fixture run: status `succeeded`, response exactly `MOSAIC_HELLO_OK`, mission snapshot recorded.
+  - Run records written once under `<dataRoot>/runs/r-<utcstamp>-<rand>/`; reruns never clobber.
+- Failure or correction: none this phase.
+- Credential check: no credential material in task data, run records, or logs.
+
+## Result (M2)
+
+Mission/task layer verified end-to-end. `main` merged with M2 and tagged `mission-task-v1`.
+