Introduces .gemini/commands/ with TOML definitions for 7 slash commands
(build, spec, planning, review, test, ship, code-simplify), enabling
Gemini CLI to auto-discover them from the project root.
The planning command is named /planning rather than /plan to avoid
conflicting with a Gemini CLI internal command name.
Updates README.md project structure tree and adds a Slash Commands
section with a reference table to docs/gemini-cli-setup.md.
CooperSheroy