[codex] Prefer Carbon desktop env vars

[w287346141]

Summary

• let the desktop RPC launcher prefer CARBONCODE_CLI while keeping REASONIX_CLI as a legacy fallback
• let debug builds open DevTools with CARBONCODE_DEVTOOLS while keeping REASONIX_DEVTOOLS compatibility
• add productization coverage so desktop developer env vars do not regress back to Reasonix-only names
• add lightweight Tauri Node resource placeholders so local Rust checks can validate the desktop shell
• run rustfmt over the touched Rust files

Why

The desktop surface is already Carbon-branded, but a couple of developer env hooks still required Reasonix names. This keeps old local workflows working while making Carbon Code the primary interface.

The Tauri config also expects platform Node resource paths to exist. The committed placeholders satisfy local resource validation; release packaging replaces them with real Node binaries.

Validation

• npm.cmd test -- --run tests/carbon-productization.test.ts
• npm.cmd run verify
• cargo fmt --check
• cargo check