Add sandboxed development mode via CODING_GATEWAY_CONFIG_HOME env var

[datasciencemonkey]

Problem

Developers working on coding-gateway risk mangling their own Claude Code, Codex, and Gemini CLI configurations when testing the tool locally. There is no safe way to develop and test without affecting real config files.

Solution

Add a CODING_GATEWAY_CONFIG_HOME environment variable that redirects all config reads/writes to an alternate directory. When set, all paths (~/.claude/, ~/.codex/, ~/.gemini/, ~/.coding-gateway/) resolve relative to this directory instead of $HOME.

Include a Makefile with sandbox-aware dev commands (make configure, make run TOOL=claude, make inspect, make clean).

Acceptance Criteria

• CODING_GATEWAY_CONFIG_HOME=/tmp/sandbox coding-gateway configure writes to /tmp/sandbox/.claude/ etc.
• Default behavior (no env var) is unchanged — uses real $HOME
• Makefile with dev commands that automatically use sandbox
• Documented in project CLAUDE.md