feat: add fx agent support

[rizvancodes]

Summary

• add an fx harness backed by fx ask --json
• support model discovery for Gateway and subscription catalogs, model selection, saved sessions, and continuation
• force FX_PERMISSION_MODE=ask and document that existing fx allow rules still apply
• treat reasoning as managed by fx

Testing

• cargo fmt --check
• cargo test --locked
• cargo clippy --locked --all-targets -- -D warnings
• live test with fx 0.0.5 and a Codex subscription: one-shot ask and session continuation through ask -c