cli: saving preferences replaces symlinked cli.json

Summary

Saving CLI preferences replaces a symlinked ~/.config/opencode/cli.json with a regular file. This breaks dotfile management tools such as GNU Stow and causes the live preferences to diverge from the version-controlled target.

Environment

• opencode version: opencode2 v0.0.0-beta-18219
• OS: Ubuntu Linux 6.8.0-124-generic x86_64
• Terminal: TERM=xterm-256color
• Shell: /usr/bin/zsh
• Install/channel: npm beta, managed by mise
• Active plugins: ~/.config/opencode/plugins/herdr-agent-state.js, @plannotator/opencode@latest

Reproduction

1. Store cli.json in a dotfiles repository and symlink it to ~/.config/opencode/cli.json (for example, with GNU Stow).
2. Open OpenCode v2 and change a CLI preference such as the theme.
3. Inspect ~/.config/opencode/cli.json with ls -l or test -L.

Expected Behavior

OpenCode should preserve the symlink and update its target, or otherwise avoid replacing the user-managed configuration path.

Actual Behavior

OpenCode removes the symlink and creates a regular ~/.config/opencode/cli.json containing the updated preferences. The Stow-managed target remains unchanged, so subsequent dotfile reconciliation reports a conflict or must preserve the new file and recreate the symlink.

Additional Context

This reproduces when changing preferences through the v2 CLI UI. A reconciliation adapter currently works around it by diffing and backing up the replacement file, removing it, and letting Stow restore the symlink.

Example before changing a preference:

~/.config/opencode/cli.json -> ../../.dotfiles/stow/opencode/.config/opencode/cli.json

After changing a preference, cli.json is a regular file rather than that symlink. This is likely caused by an atomic write implemented as writing a temporary file and renaming it over cli.json; symlink-aware persistence would avoid disconnecting the managed target.