docs: add roster building example to llms.txt

Show bvMin, bvMax, and unitType filter usage in a new example query
for LLM consumers building roster generation features.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: graylikeme

crates/api/src/handlers/llms_txt.rs

@@ -143,6 +143,26 @@ To paginate: pass `endCursor` from the previous response as `after` in the next
 }}
 ```
 
+### Filter units by BV range and unit type (roster building)
+```graphql
+{{
+  units(first: 20, bvMin: 1000, bvMax: 2000, unitType: "mech", eraSlug: "clan-invasion", factionSlug: "clan-wolf") {{
+    edges {{
+      node {{
+        slug
+        fullName
+        tonnage
+        bv
+        role
+      }}
+    }}
+    pageInfo {{
+      totalCount
+    }}
+  }}
+}}
+```
+
 ### Filter units by faction and era
 ```graphql
 {{