test(adapters): seam selftests — deterministic mock cases + mission injection (#18)

- test-config: absent adapter defaults to pi; mock validates; unknown
  adapter exits 2; env exports MOSAIC_ADAPTER (24 cases total)
- test-task: mock adapter gate pass/mismatch (no provider needed),
  mismatch reason asserted, unknown adapter fails closed, mission section
  injected into generated prompt asserted by content (24 cases total)
- harness fixes: helpers defined before use; per-case config files (no
  cross-case leakage); newest-run selection for the live case; deduped
  accidentally duplicated live block

Closes #18

README.md

@@ -101,6 +101,24 @@ scripts/test-release.sh                       # release selftests
 
 A failed health check never activates; the previously active release remains deployed. Updating the software therefore cannot corrupt the running installation: package beside, gate, then flip. Verified by the update/refusal/rollback drills in BUILD-LOG Phase 7.
 
+## Runtime adapters (M4)
+
+The harness boundary is formalized: everything upstream (config, contracts, missions, tasks, run records) is harness-agnostic; everything inside an adapter belongs to one runtime.
+
+```text
+adapters/<name>/adapter.sh   env in: MOSAIC_SYSTEM_PROMPT_FILE, MOSAIC_REQUEST,
+                             MOSAIC_PROVIDER, MOSAIC_MODEL
+                             stdout: response only; stderr: diagnostics
+```
+
+- Selection: `execution.adapter` in config.json (optional; `pi` default; allowlist `pi`, `mock`)
+- `pi` — pinned Pi CLI, noninteractive print mode, ambient discovery off
+- `mock` — deterministic test adapter; never for real verification
+- Mission directives have a sanctioned injection point: when a task references a mission, the task runner mounts the run snapshot and the generated prompt gains a `MISSION (runtime)` section (objective + directives) after the four immutable contracts
+- Adding a harness (Claude, Codex, OpenCode) later means adding one directory — no orchestrator changes
+
+See `adapters/README.md` for the full contract.
+
 See `docs/plans/2026-09-02_atomic-mosaic-foundation.md` for the full plan.
 
 Inside the container:
@@ -137,7 +155,7 @@ 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/release.sh     # package / activate / rollback / status (see Release model)
 scripts/test-config.sh # fast config-layer selftests (no Docker)
-scripts/test-task.sh   # mission/task selftests (schema + live runs)
+scripts/test-task.sh   # mission/task selftests (schema + adapter seam + live runs)
 scripts/test-release.sh # release selftests
 scripts/reset.sh       # delete the configured data root (safety-checked)
 ```