docs: refine downstream onboarding workflow

docs/export-manifest.md

@@ -14,8 +14,19 @@ Use this manifest with:
   consciously reapply any intentional downstream divergence
 - `manual-merge`: always review and merge with downstream state
 - `downstream-owned`: do not overwrite during template upgrades
-- `do-not-export`: keep in the template repo only unless the Operator explicitly
-  chooses otherwise
+- `do-not-export`: keep out of downstream live files and out of the vendored
+  `.minions-template/` snapshot unless the Operator explicitly chooses
+  otherwise
+
+## Vendored Snapshot Rule
+
+`.minions-template/` is an export-ready snapshot, not a full clone of the
+template repo.
+
+- exclude `.git/`
+- exclude all `do-not-export` files
+- keep only the files needed to onboard and later upgrade downstream minion
+  workflow
 
 ## Manifest