feat(mcp): per-query @mcp(...) annotation + per-param @description + @instruction folding

Wire the `.gq` authoring surface that controls how a stored query is projected
as an MCP tool. All of it rides in the query source (content-addressed,
re-parsed at boot), so there is no cluster.yaml / catalog / serving-snapshot
plumbing — and it is orthogonal to Cedar `invoke_query` (presentation, not
authorization).

- Per-parameter `@description("…")` (leading the variable) → carried on
  `Param.description`, mapped through `param_descriptor`, and emitted on the
  outer JSON-Schema property by `param_json_schema`, so it shows up in both the
  MCP tool input schema and the `GET /queries` catalog.
- Query `@mcp(expose: <bool>, tool_name: "<name>")` → parsed into
  `QueryDecl.mcp`; `StoredQuery::is_exposed()` / `effective_tool_name()` resolve
  from it. `expose: false` hides a query from the agent surface (`tools/list`,
  `stored_query_list`, run-by-name) while keeping it HTTP/service-callable.
- `@instruction` is folded into the MCP tool description (after `@description`),
  so the agent-facing how/when-to-use guidance reaches `tools/list`.
- Removes the now-dead `RegistrySpec.{expose, tool_name}` fields (server + CLI);
  `settings.rs` no longer hardcodes `expose: true`. Test helpers express
  exposure by injecting `@mcp(expose: false)` into the source (the real path).

openapi.json regenerated: `ParamDescriptor` gains an optional `description`.

Tests: compiler parser (param @description, @mcp parse + duplicate rejection),
api-types schema_equivalence (description on the outer property), server mcp
(folded description + param docs + @mcp tool rename, list==call). Full
workspace gate green.

crates/omnigraph-cli/src/helpers.rs

@@ -780,8 +780,6 @@ fn registry_from_serving_queries(
         .map(|q| omnigraph_server::queries::RegistrySpec {
             name: q.name.clone(),
             source: q.source.clone(),
-            expose: false,
-            tool_name: None,
         })
         .collect();
     QueryRegistry::from_specs(specs).map_err(|errors| {
@@ -890,8 +888,8 @@ pub(crate) async fn execute_queries_list(
             .iter()
             .map(|q| QueriesListItem {
                 name: q.name.clone(),
-                mcp_expose: q.expose,
-                tool_name: q.tool_name.clone(),
+                mcp_expose: q.is_exposed(),
+                tool_name: q.decl.mcp.tool_name.clone(),
                 mutation: q.is_mutation(),
                 params: q
                     .decl