[codex] Prefer Carbon Code environment variables

[w287346141]

Summary

• prefer CARBONCODE_* environment variables for dashboard host/token, theme, startup profiling, URL opening, and flush interval controls
• keep legacy REASONIX_* variables as fallbacks so existing setups continue to work
• update theme picker copy to mention CARBONCODE_THEME
• add tests for CARBONCODE_THEME, CARBONCODE_PROFILE_STARTUP, and CARBONCODE_NO_OPEN behavior

Why

Carbon Code still exposed several Reasonix-branded runtime knobs. This keeps compatibility while making the documented/default environment variable surface match the product name.

Validation

• npm.cmd test -- --run tests/startup-profile.test.ts tests/slash.test.ts tests/open-url.test.ts tests/config.test.ts
• npm.cmd run typecheck
• npm.cmd run lint
• npm.cmd run verify

[Yapie0]

Closing as superseded. The useful CARBONCODE_* env migration pieces have been merged through the smaller focused PRs (#13, #34, #35, #36, #38, #39 and related follow-ups). This larger batch now conflicts with main and would duplicate already-validated changes.