fix(mcp): make explore guidance advisory (#1504) (#1805)

Apply upstream commit 1572d90d7174f0c65019e768c8f65728fae668c5 from
PR #1570 by @rongbc.

Clarify that suggested explore call counts are advisory and extra calls
remain available. Sync the MCP initialization guidance and eval probe,
retain the upstream regression tests, close the footer's bold formatting,
and credit the fix under Unreleased in the changelog.

Validation: npm run build; focused Vitest (3 files, 53 tests); source and
dist wording greps; git diff --check.

Fixes #1504

Co-authored-by: Colby McHenry <colbymchenry@users.noreply.github.com>

scripts/agent-eval/probe-suite-envelope.mjs

@@ -85,7 +85,7 @@ try {
       epilogueCut: text.includes('omitted for size'),
       sectionCut: text.includes('output truncated to budget'),
       notShown: text.includes('Not shown above'),
-      budgetNote: text.includes('**Explore budget:'),
+      budgetNote: text.includes('advisory only, NOT a quota'),
     });
   }
 } finally {