Contextualize/reinforce importance of chatmodes/prompts/instructions in system prompt #257152
Description
Right now system prompt has little knowledge about customizations (instructions, prompts, modes); they are added mostly with a one-line prefix.
Codex has some strong language around AGENTS.md that suggests a much stronger adherence.
# AGENTS.md spec
- Some examples might be: coding conventions, info about how code is organized, or instructions for how to run or test code.
- AGENTS.md files may provide instructions about PR messages (messages attached to a GitHub Pull Request produced by the agent, describing the PR). These instructions should be respected.
- Instructions in AGENTS.md files:
…
- For every file you touch in the final patch, you must obey instructions in any AGENTS.md file whose scope includes that file.
- Instructions about code style, structure, naming, etc. apply only to code within the AGENTS.md file's scope, unless the file states otherwise.
- More-deeply-nested AGENTS.md files take precedence in the case of conflicting instructions.
- Direct system/developer/user instructions (as part of a prompt) take precedence over AGENTS.md instructions.
- If the AGENTS.md includes programmatic checks to verify your work, you MUST run all of them and make a best effort to validate that the checks pass AFTER all code changes have been made.
- This applies even for changes that appear simple, i.e. documentation. You still must run all of the programmatic checks.