feat(xce-harness-component): xce-test-multihost builds/checks harness explicitly

run-multihost-verify.sh's XT default now points at
\$BUILD_DIR/harness/xce_test and fails fast (actionable "run
/xce-build harness" message) if missing. xce_test is the sole
mechanism (xtest()) by which the script's job-scenario functions
talk to the cluster, so a hard failure is correct, matching task
3.2's already-approved precedent.

SKILL.md's Prerequisites, env-var docs, and Dependencies sections
updated to match.

Verified live: sourced the script (main() guarded from running),
called precondition_checks() with the binary present and then moved
aside, confirmed the exact failure message, restored the binary
(checksum-verified intact).

Task 3.4 of xce-harness-component.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RLSNJhNoULvVcUhXEFafQC

.kiro/specs/xce-harness-component/tasks.md

@@ -49,7 +49,7 @@
   - _Requirements: 5.2, 5.4_
   - _Boundary: xce-deploy-ha skill_
   - _Depends: 3.1_
-- [ ] 3.4 (P) Update the multi-host verification workflow's harness prerequisite
+- [x] 3.4 (P) Update the multi-host verification workflow's harness prerequisite
   - Update `.claude/skills/xce-test-multihost/run-multihost-verify.sh` and `SKILL.md`
   - Observable: running the workflow from a build dir where only controller was built either builds the harness itself or reports a clear, actionable message
   - _Requirements: 5.3, 5.4_