fix(tests): resolve the prod binary from CBM_TEST_BINARY, not a hardcoded build/c

The Linux legs of the local 3-OS ladder build into build/linux-arm64 /
build/linux-amd64, not build/c, so Step 5c died with
'missing binary: /src/build/c/codebase-memory-mcp' and took the whole
leg down. Steps 5 and 5b already pass CBM_TEST_BINARY (derived from
$BUILD_DIR) to their scripts; 5c did not, and its test hardcoded the
path.

CI never caught this because every CI leg uses the default BUILD_DIR of
build/c - the container legs are the only ones that differ, which is
precisely what the local ladder is for.

- test_worker_error_response.sh honours ${CBM_TEST_BINARY:-build/c/...}
  like its sibling watchdog tests; the default keeps bare manual runs working
- scripts/test.sh passes CBM_TEST_BINARY to Step 5c, matching 5 and 5b
- test_hook_conflict_notice.sh carried the identical hardcoding and is
  fixed the same way (local-only today, but wrong is wrong)

Verified: with the fix the test passes against an out-of-tree binary;
reverting the fix reproduces the ladder's exact failure (rc=2,
'missing binary: <worktree>/build/c/codebase-memory-mcp').

Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>

scripts/test.sh

@@ -280,7 +280,7 @@ CBM_TEST_BINARY="$WATCHDOG_BINARY" bash "$ROOT/tests/test_worker_watchdog.sh"
 # still exits nonzero for the user-facing tool error, but the supervisor must
 # preserve that response instead of misreporting exit_nonzero as a file crash.
 echo "=== Step 5c: worker error-response transport regression ==="
-bash "$ROOT/tests/test_worker_error_response.sh"
+CBM_TEST_BINARY="$WATCHDOG_BINARY" bash "$ROOT/tests/test_worker_error_response.sh"
 
 # Step 5d (#1388) is DELIBERATELY NOT GATING HERE — see
 # tests/test_hook_conflict_notice.sh for the full what-was-tried record.