Make invoke_query graph-scoped (one branch authority)

invoke_query gates reaching the curated stored-query surface — a graph-level
capability. Per-branch/snapshot access is already enforced by the inner
read/change gate in run_query/run_mutate (authorized against the resolved
branch), so branch-scoping the outer gate was redundant AND wrong for snapshot
reads (it defaulted to main). Drop the branch dimension: remove InvokeQuery
from uses_branch_scope (it joins admin as graph-scoped) and authorize the
boundary gate with branch: None.

Lossless: an actor confined to branch X by their read/change rules can still
only invoke a stored query that touches X. A rule that sets branch_scope on
invoke_query is now rejected by validate() — write invoke_query in its own
rule.

Ripple (atomic): restructure the server invoke fixture so invoke_query sits in
its own branch_scope-free rule; invert invoke_query_is_branch_scoped ->
invoke_query_rejects_branch_scope; the per-graph authorize test uses
branch: None; docs (policy.md, server.md, the InvokeQuery doc). No wire/OpenAPI
change.

crates/omnigraph-server/src/lib.rs

@@ -2213,7 +2213,11 @@ async fn server_invoke_query(
         handle.policy.as_deref(),
         PolicyRequest {
             action: PolicyAction::InvokeQuery,
-            branch: req.branch.clone().or_else(|| Some("main".to_string())),
+            // Graph-scoped: no branch dimension. The per-branch/snapshot
+            // access is enforced by the inner read/change gate in the
+            // runner, so the outer gate must not resolve a branch (doing so
+            // was wrong for snapshot reads).
+            branch: None,
             target_branch: None,
         },
     )