docs: add Ag Advisor grain-marketing prompt library

Example end-user prompts (planning + monitoring) for the downstream Drawbar Ag
Advisor, each mapped to the ag-bids MCP/API tools that answer it, plus
cross-cutting synthesis examples. Linked from the README.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -95,3 +95,7 @@ auto-updates it every 5 minutes.
 
 For the full per-release detail (signatures, conventions, example question → call
 mappings), see [CHANGELOG.md](CHANGELOG.md).
+
+Grain-marketing prompt library for the downstream Ag Advisor (planning +
+monitoring, with the tools each prompt uses):
+[examples/ag-advisor-prompts.md](examples/ag-advisor-prompts.md).