#462 fix(cliproxyapi): read API key from .env and skip dotfiles sync on Linux

Summary

• Read CLIPROXY_API_KEY from .env file for API authentication on Linux
• Skip redundant auth file sync to ~/dotfiles/objectstore/auths/ on Linux (macOS behavior preserved)

Changes

API Key from .env

• Source .env file to get CLIPROXY_API_KEY
• Enable API key authentication only when the key is set

Skip dotfiles sync on Linux

• start.sh: Bootstrap from dotfiles is now Darwin-only
• backup-auth.sh: Sync to dotfiles is now Darwin-only
• default.nix: Removed dotfiles path from Linux systemd path watcher

On Linux, auth files now only sync between:

• R2 cloud storage
• ~/.cli-proxy-api/objectstore/auths/
• ~/.ccs/cliproxy/auth/

Test plan

• make format passes
• make shell-test passes (242 examples, 0 failures)
• Verified dotfiles/objectstore/auths stays empty after backup service runs on Linux

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