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

scripts/test-config.sh

@@ -12,6 +12,10 @@ trap 'rm -rf "$SANDBOX"' EXIT
 PASS=0
 FAIL=0
 
+check() {
+    if [ "$2" = "0" ]; then PASS=$((PASS+1)); echo "ok   $1"; else FAIL=$((FAIL+1)); echo "FAIL $1"; fi
+}
+
 # expect_exit NAME EXPECTED_RC -- command...
 expect_exit() {
     local name="$1" expected="$2"