docs: add Codex support plan

[netopsengineer]

Motivation

• Provide a structured, gated plan to add Codex as a second backend while preserving Claude as the default and maintaining current CLI/output contracts.
• Capture a capability spike to verify whether codex exec or app-server exposes a stable, machine-readable skill-selection signal before implementing a Codex transport.
• Define provider-neutral backend boundaries and ensure additive compatibility with existing invocations and reporting fields.
• Surface risks, stop conditions, and a phased delivery sequence to avoid regressing current behavior.

Description

• Add docs/CODEX_SUPPORT_PLAN.md containing a detailed multi-phase integration plan, decision gate criteria, and a suggested internal Backend protocol.
• Specify a spike procedure to validate codex exec/app-server event schemas and the requirement that candidate selection be signaled without relying on natural-language output.
• Define phase-level changes including refactoring to src/skill_optimizer/backends/ (protocol + claude/codex adapters), evaluation/improvement flows, CLI additions like --backend, and output fields such as backend and estimated_agent_calls.
• Enumerate required tests, mocked fixtures, compatibility contracts, and release gates to ensure parity and isolation across backends.

Testing

• No automated tests were added or run for this change because it only introduces documentation in docs/CODEX_SUPPORT_PLAN.md.
• The plan includes a comprehensive test matrix to be implemented in later phases, including recorded JSONL fixtures and fake-binary integration tests.

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Codex Task