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-server/tests/support/mod.rs

@@ -145,14 +145,24 @@ pub fn graph_path(root: &Path) -> PathBuf {
 }
 
 pub fn stored_query_registry(specs: &[(&str, &str, bool)]) -> QueryRegistry {
+    // MCP `expose` now lives in the `.gq` source `@mcp(...)` annotation. The
+    // `(name, source, expose)` tuple stays for ergonomics: when `expose` is
+    // false, inject `@mcp(expose: false)` between the param-list `)` and the
+    // body `{` (the first `{` in a query source is always the body open), so
+    // the real parse path is exercised.
     QueryRegistry::from_specs(
         specs
             .iter()
-            .map(|(name, source, expose)| RegistrySpec {
-                name: name.to_string(),
-                source: source.to_string(),
-                expose: *expose,
-                tool_name: None,
+            .map(|(name, source, expose)| {
+                let source = if *expose {
+                    source.to_string()
+                } else {
+                    match source.find('{') {
+                        Some(i) => format!("{}@mcp(expose: false) {}", &source[..i], &source[i..]),
+                        None => source.to_string(),
+                    }
+                };
+                RegistrySpec { name: name.to_string(), source }
             })
             .collect(),
     )