test(hooks): always run the session-start payload test, no jq skip guard

- Problem: the CI step skipped the session-start hook test when jq was
  missing, turning the whole step into a silent no-op on runners without
  jq — the same silently-green shape this PR exists to remove.
- Fix: run hooks/session-start-test.sh unconditionally. The script already
  detects jq itself (HAS_JQ) and asserts the hookSpecificOutput envelope on
  both code paths, so a jq-less runner now covers the hook's real
  jq-missing branch instead of skipping it.
- Verification: test passes with jq present; with jq removed from PATH
  (minimal PATH env, command -v jq empty) the hook emits the jq-is-required
  envelope and the test still passes; workflow YAML parses clean.

.github/workflows/test-plugin-install.yml

@@ -41,12 +41,7 @@ jobs:
         run: node --test scripts/validate-reference-links-test.js
 
       - name: Test session-start hook payload
-        run: |
-          if ! command -v jq >/dev/null 2>&1; then
-            echo "jq not found; skipping session-start hook test"
-            exit 0
-          fi
-          bash hooks/session-start-test.sh
+        run: bash hooks/session-start-test.sh
 
   validate-commands:
     name: Validate command parity and description sync