Fix documentation inconsistency #1132

[Rish-it]

Documentation: Clarify instruction file usage and resolve README inconsistencies (#1132)

This update resolves inconsistencies between README.md and examples/prompting_guide.md related to instruction file names and expected locations.

Changes

• Global instructions path: Updated README.md to correctly reference ~/.codex/instructions.md instead of the outdated ~/.codex/AGENTS.md, aligning with actual CLI behavior.
• Project instruction file naming: Standardized usage of AGENTS.md as the preferred file for project-specific instructions across both README.md and examples/prompting_guide.md.
• Legacy file support: Added clarification that CODEX.md, codex.md, and .codex.md are still supported for backward compatibility, but AGENTS.md is recommended for new projects.
• CLI help text: Revised the --no-project-doc flag description in src/cli.tsx to reflect support for multiple project documentation file names (e.g., AGENTS.md, CODEX.md, etc.).

These changes improve clarity and ensure the documentation reflects the current behavior of the CLI.

[github-actions]

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[Rish-it]

I have read the CLA Document and I hereby sign the CLA

[Rish-it]

Hey @bolinfest, Could you please take a look at PR #1141 I've addressed the documentation inconsistencies mentioned in issue #1132

Thanks!