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

README.md

@@ -61,6 +61,28 @@ Rules enforced by `scripts/mosaic-config.mjs`:
 
 Run paths (`build/hello/verify/reset`) fail closed when configuration is missing or invalid; they never invent it.
 
+## Missions & tasks (M2)
+
+Missions and tasks are validated JSON data (strict schemas, version-pinned). The M2 layer is host-side only: mission directives are recorded for provenance but do not yet reach the runtime system prompt (capability/policy layer comes later).
+
+```text
+missions/hello.json          objective + directives (missionVersion 1)
+tasks/hello-marker.json      prompt + optional mission ref + expectExact + timeout
+<dataRoot>/runs/r-<id>/      immutable run record: task.json, mission.json,
+                             stderr.txt, result.json (all write-once)
+```
+
+Usage:
+
+```bash
+scripts/run-task.sh validate tasks/hello-marker.json   # strict validation, writes nothing
+scripts/run-task.sh run tasks/hello-marker.json        # execute; result recorded under dataRoot/runs
+scripts/mosaic-task.mjs list                           # list runs and statuses
+scripts/test-task.sh                                   # selftests (schema negatives + live runs)
+```
+
+A run exits 0 only when its expectation is met (`expectExact` match); mismatches, nonzero agent exits, and timeouts record `status: failed` in `result.json` and exit 1. Each run gets a unique directory — rerunning never rewrites history.
+
 See `docs/plans/2026-09-02_atomic-mosaic-foundation.md` for the full plan.
 
 Inside the container:
@@ -93,7 +115,9 @@ scripts/bootstrap.sh   # create config.json if absent (idempotent)
 scripts/build.sh       # build the image
 scripts/hello.sh       # one-shot request; prints the model response
 scripts/verify.sh      # full gated test; exit 0 only on exact MOSAIC_HELLO_OK
+scripts/run-task.sh    # run a mission/task file (see Missions & tasks)
 scripts/test-config.sh # fast config-layer selftests (no Docker)
+scripts/test-task.sh   # mission/task selftests (schema + live runs)
 scripts/reset.sh       # delete the configured data root (safety-checked)
 ```