[codex] Fix setup.sh overwriting Claude Code settings

[happy-pills-dev]

What changed

Fixes the Claude Code setup path in setup.sh so it no longer overwrites an existing ~/.claude/settings.json.

The installer now:

• creates settings.json when it does not exist
• backs up an existing settings file before modifying it
• preserves unrelated Claude Code settings and existing MCP servers
• only adds or updates mcpServers.openbrain
• refuses to overwrite invalid/non-object JSON or an invalid mcpServers shape

Why

Option 3 previously used cat > "$CONFIG", replacing the full Claude Code settings file and potentially deleting user permissions, existing MCP servers, or other configuration. That matches the data-loss behavior reported in #13.

Validation

• bash -n setup.sh
• focused Git Bash test that merges into an existing settings file while preserving unrelated keys and existing MCP servers
• focused Git Bash test that leaves invalid JSON untouched and creates a backup before failing safely

Closes #13