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

run-ha-soak.sh's XT default now points at src/xce.build/harness/xce_test
(where /xce-build harness places it) and fails fast (exit 2, actionable
"run /xce-build harness" message) if missing, rather than assuming a
controller build produced it. xce_test is the soak's own driver client
(~30 call sites: job submit/verify, walcompare, waldump, snapdiff,
worker join/cancel), so a hard failure is correct, not a soft-skip.

SKILL.md's build instructions, goal-loop rebuild step, and
troubleshooting table updated to mention building harness, with
--incremental guidance correctly conditioned on build-dir-clobbering
semantics (bug-1565) rather than blanket-recommended.

Verified live: moved the binary aside, ran the script, confirmed the
exact failure message, restored the binary (checksum-verified intact).

Task 3.2 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

@@ -37,7 +37,7 @@
   - In `src/tools/xce-build.sh`, drop `controller/xce_test` from the `controller)` case's compile target list; add a new `harness)` case following the identical three-part `setup_build` / `meson compile` / conditional `meson test --suite harness` template already used by `scheduler)`/`worker)`
   - Observable: `/xce-build harness` on a fresh build dir (no prior controller build) produces the `xce_test` binary and runs the `harness` suite; `/xce-build controller` alone does not produce `xce_test`
   - _Requirements: 2.1, 2.2, 2.3, 2.4_
-- [ ] 3.2 (P) Update the HA soak workflow's harness prerequisite
+- [x] 3.2 (P) Update the HA soak workflow's harness prerequisite
   - Update `.claude/skills/xce-test-ha/run-ha-soak.sh` and `SKILL.md` so the soak ensures the harness is built (or fails with a clear message) instead of assuming a controller build produced it
   - Observable: running the workflow from a build dir where only controller was built either builds the harness itself or reports a clear, actionable message instead of a raw spawn/connect failure
   - _Requirements: 5.1, 5.4_