fix: prevent studio client-context override from breaking Claude TTS

[launsion-boop]

Summary

• fix a client-context leak caused by src/studio/server.ts
• move Studio-only ECHOCODING_CLIENT=default override from module top-level into startStudio()
• keep Studio writing global config while avoiding side effects on normal CLI commands (status/say/ask)

Bug

When the CLI imported Studio, top-level env mutation forced runtime client to default, so Claude sessions could route say/ask to the wrong daemon and appear as hook-only audio.

Validation

• npm test passes (16/16)
• verified ECHOCODING_CLIENT=claude npx echocoding status reports Client context: claude
• verified say triggers TTS mp3 playback in Claude client context

⚠️ Review bypassed — reason: trivial single-file bugfix: prevent studio client env leak into CLI runtime (approved by boss)