refactor(config): remove top-level policy:/queries: under v1

Top-level `policy:`/`queries:` (the single-graph-mode blocks) are removed under
`version: 1` — policy and stored queries belong on the owning `graphs.<name>`
entry. Both are rejected at load (pointing at the per-graph block) and honored-
but-warned under the legacy schema; the per-graph `graphs.<name>.policy`/`queries`
blocks are nested, so the key scan leaves them untouched. Drops the now-illegal
empty top-level `policy: {}` from the shared CLI test helpers.

crates/omnigraph-cli/tests/support/mod.rs

@@ -129,7 +129,6 @@ defaults:
 query:
   roots:
     - .
-policy: {{}}
 ",
         yaml_string(&graph.to_string_lossy())
     )
@@ -151,7 +150,6 @@ defaults:
 query:
   roots:
     - .
-policy: {{}}
 ",
         yaml_string(url)
     )