feat(admin-config-tool): wire xceadmin into HA deploy/test skills

deploy-ha.sh's --build-config path (task 4.2) pointed at
src/xceadmin/.venv/bin/xceadmin, broken by today's meson/nix rewiring
(xceadmin no longer builds a venv there). Fixed to search the meson
build dir (xce.build/build.release/build.dbg), a nix-installed
result/bin, then PATH.

/xce-deploy-ha's SKILL.md never mentioned xceadmin or --build-config
(predates task 4.2) — documented both.

/xce-test-ha's run-ha-soak.sh hard-required a pre-existing cluster.bin
under ha/data/, exit 2 if missing, with no xceadmin path at all. Now
always builds cluster.bin fresh from src/configs/*.yaml via xceadmin
before every soak run, same resolution order as deploy-ha.sh.

ha/data/cluster.bin is no longer a checked-in fixture: removed from
git tracking and gitignored. It was stale relative to xceadmin's
output anyway (different hash from a different, older generation
path) -- the xceadmin-built one matches the known-good golden-fixture
hash from admin-config-tool task 5.4.

.gitignore

@@ -33,3 +33,7 @@ ucai-formatter-cache.local.json
 /final_suite_run.log
 /xce-durability-*.log
 /xce-ha-soak*.log
+
+# --- session checkpoint 2026-07-22: cluster.bin is always freshly built by
+# xceadmin now (deploy-ha.sh / run-ha-soak.sh), never a checked-in fixture ---
+ha/data/cluster.bin