Add use_agent_identity feature flag

[adrian-openai]

Summary

Stack PR 1 of 4 for feature-gated agent identity support.

This PR adds the under-development features.use_agent_identity config flag and schema plumbing only. Runtime behavior remains unchanged with the flag off.

Stack

• PR1: Add use_agent_identity feature flag #17385 - this PR
• PR2: Register agent identities behind use_agent_identity #17386 - register agent identities when enabled
• PR3: Register agent tasks behind use_agent_identity #17387 - register agent tasks when enabled
• PR4: Use AgentAssertion downstream behind use_agent_identity #17388 - use AgentAssertion downstream when enabled

Validation

Covered as part of the local stack validation pass:

• just fmt
• cargo test -p codex-core --lib agent_identity
• cargo test -p codex-core --lib agent_assertion
• cargo test -p codex-core --lib websocket_agent_task
• cargo test -p codex-api api_bridge
• cargo build -p codex-cli --bin codex

Notes

The full local app-server E2E path is still being debugged after PR creation. The current branch stack is directionally ready for review while that follow-up continues.

[adrian-openai]

For continuity, this replaces the previous closed draft stack:

• Original PR1: Add use_agent_identity feature flag #15587
• Original PR2: Register agent identities behind use_agent_identity #15588
• Original PR3: Register agent tasks behind use_agent_identity #15589
• Original PR4: Use AgentAssertion downstream behind use_agent_identity #15590

I tried reopening the original PRs first, but GitHub reported they were not reopenable (viewerCanReopen=false / reopenPullRequest failed), so this new stack is the active review path.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Fix ImageDetailOriginal stage assertion

The added image_detail_original_feature_is_under_development test asserts Feature::ImageDetailOriginal.stage() == Stage::UnderDevelopment, but FEATURES defines ImageDetailOriginal as Stage::Experimental. This makes cargo test -p codex-features fail and can block CI on unrelated work.

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