[codex] Load AGENTS.md from all bound environments

[anp-oai]

Why

We already have the machinery to support multiple environments on a single thread, but we only show the model the contents of AGENTS.md files in the primary environment.

We should show the model all of the relevant project instructions when we know there's more than one environment.

Known Gaps

As discussed in the RFC, this implementation:

1. doesn't handle environments being added/removed to/from the thread after its creation
2. it doesn't enforce an aggregate context budget across environments, and instead applies the configured project maximum independently to each environment

Implementation

• Discover project instructions in environment order with an independent byte budget per environment and preserve source provenance/order.
• Keep the legacy fragment byte-for-byte when exactly one environment contributes project instructions; use environment-labeled sections when two or more environments contribute.
• Freeze the complete rendered fragment in LoadedAgentsMd, insert it directly into requests, and recognize both layouts in contextual and memory filtering.
• Add exact rendering, independent-budget, source-order, creation-snapshot, and consumer coverage without changing app-server schemas.

[chatgpt-codex-connector]

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