Nudge models to use fields param in server instructions

Add a short line to the Context management section of the server
instructions encouraging models to use the 'fields' parameter, when a
tool offers it, to request only the fields they need. Sits alongside the
existing minimal_output guidance and keeps the addition minimal to avoid
growing context usage.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: tommaso-moro

pkg/inventory/instructions.go

@@ -24,6 +24,7 @@ Tool selection guidance:
 Context management:
 	1. Use pagination whenever possible with batches of 5-10 items.
 	2. Use minimal_output parameter set to true if the full information is not needed to accomplish a task.
+	3. Use the 'fields' parameter, when a tool offers it, to request only the fields you need and reduce response size.
 
 Tool usage guidance:
 	1. For 'search_*' tools: Use separate 'sort' and 'order' parameters if available for sorting results - do not include 'sort:' syntax in query strings. Query strings should contain only search criteria (e.g., 'org:google language:python'), not sorting instructions.`