Restack context fragment standardization

Reapply the standardized model-visible context fragment work on top of the latest origin/main as a clean squashed restack.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/app-server-protocol/schema/json/v2/ThreadStartParams.json

@@ -123,6 +123,7 @@
       ]
     },
     "developerInstructions": {
+      "description": "Custom developer override for this thread session. Takes precedence over `~/.codex/config.toml` `developer_instructions`.",
       "type": [
         "string",
         "null"