rename spawn_csv feature flag to enable_fanout (#14475)

## Summary
- rename the public feature flag for `spawn_agents_on_csv()` from
`spawn_csv` to `enable_fanout`
- regenerate the config schema so only `enable_fanout` is advertised
- keep the behavior the same: enabling `enable_fanout` still pulls in
`multi_agent`

## Notes
- this is a hard rename with no `spawn_csv` compatibility alias
- the internal enum remains `Feature::SpawnCsv` to keep the patch small

## Testing
- `cd codex-rs && just fmt`
- `cd codex-rs && cargo test -p codex-core` (running locally;
`suite::agent_jobs::*` and rename-specific coverage passed so far)

codex-rs/core/config.schema.json

@@ -363,6 +363,9 @@
             "enable_experimental_windows_sandbox": {
               "type": "boolean"
             },
+            "enable_fanout": {
+              "type": "boolean"
+            },
             "enable_request_compression": {
               "type": "boolean"
             },
@@ -459,9 +462,6 @@
             "skill_mcp_dependency_install": {
               "type": "boolean"
             },
-            "spawn_csv": {
-              "type": "boolean"
-            },
             "sqlite": {
               "type": "boolean"
             },
@@ -1871,6 +1871,9 @@
         "enable_experimental_windows_sandbox": {
           "type": "boolean"
         },
+        "enable_fanout": {
+          "type": "boolean"
+        },
         "enable_request_compression": {
           "type": "boolean"
         },
@@ -1967,9 +1970,6 @@
         "skill_mcp_dependency_install": {
           "type": "boolean"
         },
-        "spawn_csv": {
-          "type": "boolean"
-        },
         "sqlite": {
           "type": "boolean"
         },