[codex] add Rust harness app server

[gakonst]

Summary

• Add a Rust crates/harness-server normalizer/wrapper with harness-server codex|claude-code|amp.
• Use pinned codex-app-server-protocol types from OpenAI Codex and convert Claude Code/Amp native streaming into Codex App Server V2 events.
• Add typed backend traits, per-harness modules, JSON-RPC validation, semantic agent-delta validation, and cargo integration tests.
• Preserve native streaming/steering behavior and select the real Codex app-server binary that supports --listen stdio://.
• Add the harness-development skill for future harness backend work and differential debugging.
• Remove the bash harness smoke/compare scripts; real-binary coverage now runs through ignored cargo tests.

Validation

• cargo fmt --manifest-path crates/harness-server/Cargo.toml
• cargo test --manifest-path crates/harness-server/Cargo.toml
• python3 /Users/georgios/.codex/skills/.system/skill-creator/scripts/quick_validate.py .agents/skills/harness-development
• cargo test --manifest-path crates/harness-server/Cargo.toml real_claude_code_long_streaming_is_anchored_to_native_cli -- --ignored --nocapture
• cargo test --manifest-path crates/harness-server/Cargo.toml real_amp_long_streaming_is_anchored_to_native_cli -- --ignored --nocapture
• cargo test --manifest-path crates/harness-server/Cargo.toml real_codex_long_streaming_uses_native_app_server_chunks -- --ignored --nocapture

Streaming Evidence

• Claude Code native CLI emitted 16 text stream deltas for the 24-line prompt; the wrapper emitted multiple item/agentMessage/delta chunks and the concatenated deltas exactly matched item/completed.item.text.
• Amp native CLI emitted no text stream deltas and one final assistant message for the same prompt; the wrapper now expands that final assistant text into 17 Codex item/agentMessage/delta chunks, and concatenated deltas exactly match item/completed.item.text.
• Codex native app-server streamed many small item/agentMessage/delta chunks; the wrapper pass-through preserved many chunks, and concatenated deltas exactly matched the completed item text.

Notes

• This PR is stacked on PR [codex] add Chat SDK slackbot v2 #322 (codex/slackbotv2-session-api).
• Former PR [codex] add harness development skill #324 has been folded into this PR.

[gakonst]

Closed in favor of the rebuilt branch over PR #344: #463