feat: config aliases (#18140)

Rename `no_memories_if_mcp_or_web_search` →
`disable_on_external_context` with backward compatibility

While doing so, we add a key alias system on our layer merging system.
What we try to avoid is a case where a company managed config use an old
name while the user has a new name in it's local config (which would
make the deserialization fail)

codex-rs/core/config.schema.json

@@ -863,6 +863,10 @@
           "description": "Model used for memory consolidation.",
           "type": "string"
         },
+        "disable_on_external_context": {
+          "description": "When `true`, external context sources mark the thread `memory_mode` as `\"polluted\"`.",
+          "type": "boolean"
+        },
         "extract_model": {
           "description": "Model used for thread summarisation.",
           "type": "string"
@@ -900,10 +904,6 @@
           "format": "int64",
           "type": "integer"
         },
-        "no_memories_if_mcp_or_web_search": {
-          "description": "When `true`, web searches and MCP tool calls mark the thread `memory_mode` as `\"polluted\"`.",
-          "type": "boolean"
-        },
         "use_memories": {
           "description": "When `false`, skip injecting memory usage instructions into developer prompts.",
           "type": "boolean"