[codex] Remove child AGENTS.md prompt experiment

[pakrym-oai]

Why

child_agents_md is a disabled, under-development experiment that adds a second model-visible explanation of hierarchical AGENTS.md behavior. Keeping it leaves unused prompt, configuration, documentation, and test surface.

What changed

• remove the ChildAgentsMd feature and child_agents_md config schema entry
• remove the hierarchical prompt asset, export, and instruction injection
• remove feature-specific tests and documentation
• keep the generic unstable-feature warning coverage using apply_patch_streaming_events

Normal project AGENTS.md discovery and composition are unchanged.

Testing

• just test -p codex-features
• just test -p codex-prompts
• just test -p codex-core agents_md
• just test -p codex-core unstable_features_warning

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve child_agents_md as a removed compatibility flag

When a user or wrapper still passes --enable child_agents_md, or runs --strict-config with [features].child_agents_md in config.toml, this key is now unknown because is_known_feature_key only recognizes entries from FEATURES and legacy aliases. That blocks CLI startup or strict config loading instead of treating the removed experiment as a no-op; keep a Stage::Removed registry entry/alias for child_agents_md like the other removed feature flags.

AGENTS.md reference: AGENTS.md:L103-L110

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