Merge origin/main into MR-656; retrofit + fold-in run_query

Resolves the 4 hard conflicts from PR #119 (multi-graph server mode,
MR-668) landing on main:

* `crates/omnigraph-cli/src/main.rs` imports: drop unused `ChangeRequest`,
  take main's `GraphListResponse`.
* `crates/omnigraph-server/src/api.rs`: keep branch's `ChangeRequest`
  field rename (`query_source` -> `query` with serde alias, `query_name`
  -> `name`); accept main's rustfmt.
* `crates/omnigraph-server/src/lib.rs`: take both import lists (branch's
  `QueryRequest` + main's `GraphInfo`/`GraphListResponse`); rewrite the
  `server_change` signature to combine the branch's `run_mutate`
  extraction with main's `Extension<Arc<GraphHandle>>` + `ResolvedActor`
  parameter shape.
* `docs/user/server.md`: re-apply the branch's new `/query` and `/mutate`
  rows plus deprecation notes for `/read` and `/change` on top of main's
  two-column (single-mode | multi-mode) table layout.

Auto-merged but stale callsites repaired alongside the conflict
resolutions so the merge commit compiles:

* `server_query` handler now takes `Extension(handle): Extension<Arc<GraphHandle>>`
  and `Option<Extension<ResolvedActor>>`, with policy read from
  `handle.policy.as_deref()` instead of the removed `state.policy_engine()`.

Fold-in for MR-969 (next-step seam):

* Extract `run_query` mirroring `run_mutate`: both helpers now take
  `(state, handle, actor, query: &str, name: Option<&str>,
  params_json: Option<&Value>, branch, ...)` instead of the
  `QueryRequest` / `ChangeRequest` body type. The future
  `/queries/{name}` handler can call these with registry-supplied
  fields without rebuilding the request shape.
* `server_query` / `server_read` now route through `run_query`;
  `server_mutate` / `server_change` route through `run_mutate`.
* D2 mutation rejection on `/query` is preserved via the
  `reject_mutations` flag; `/read` keeps the legacy permissive
  behavior for byte-stable back-compat.

`cargo test -p omnigraph-server --test server`: 89 passed, 0 failed.
`cargo build --workspace --tests --locked`: clean.

Refs: MR-656, MR-668, MR-969.

openapi.json

@@ -586,6 +586,63 @@
         ]
       }
     },
+    "/graphs": {
+      "get": {
+        "tags": [
+          "management"
+        ],
+        "summary": "List every graph currently registered with this server (MR-668).",
+        "description": "Multi-graph mode only. In single mode, the route returns 405 — there's\nno registry to enumerate. Cedar-gated by the server-level policy via\nthe `graph_list` action against `Omnigraph::Server::\"root\"`.\n\nOrder: alphabetical by `graph_id` (server-sorted so clients see\ndeterministic output across requests).",
+        "operationId": "listGraphs",
+        "responses": {
+          "200": {
+            "description": "List of registered graphs",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/GraphListResponse"
+                }
+              }
+            }
+          },
+          "401": {
+            "description": "Unauthorized",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ErrorOutput"
+                }
+              }
+            }
+          },
+          "403": {
+            "description": "Forbidden",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ErrorOutput"
+                }
+              }
+            }
+          },
+          "405": {
+            "description": "Method not allowed (single-graph mode)",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ErrorOutput"
+                }
+              }
+            }
+          }
+        },
+        "security": [
+          {
+            "bearer_token": []
+          }
+        ]
+      }
+    },
     "/healthz": {
       "get": {
         "tags": [
@@ -1355,6 +1412,7 @@
           "forbidden",
           "bad_request",
           "not_found",
+          "method_not_allowed",
           "conflict",
           "too_many_requests",
           "internal"
@@ -1424,6 +1482,37 @@
           }
         }
       },
+      "GraphInfo": {
+        "type": "object",
+        "description": "One entry in the response from `GET /graphs`. Cluster operators\nconsume this list to discover which graphs the server is currently\nserving. The shape is intentionally minimal — `graph_id` and `uri`\nare the only fields a routing client needs.",
+        "required": [
+          "graph_id",
+          "uri"
+        ],
+        "properties": {
+          "graph_id": {
+            "type": "string"
+          },
+          "uri": {
+            "type": "string"
+          }
+        }
+      },
+      "GraphListResponse": {
+        "type": "object",
+        "description": "Response from `GET /graphs`. Lists every graph registered with the\nserver in alphabetical order by `graph_id` (sorted server-side so\nclients get deterministic output across requests).",
+        "required": [
+          "graphs"
+        ],
+        "properties": {
+          "graphs": {
+            "type": "array",
+            "items": {
+              "$ref": "#/components/schemas/GraphInfo"
+            }
+          }
+        }
+      },
       "HealthOutput": {
         "type": "object",
         "required": [