CLI UX and discoverability

[httpdss]

Summary
Make first-run and daily usage smoother with additional commands and completions.

Proposed changes

• Add struct init to scaffold a custom structures repo with examples and docs.
• Add struct completion install [bash|zsh|fish] to emit the exact shell commands for enabling completion.
• Standardize output mode naming across CLI and MCP (console/files or console/file) and update docs.

Files

• struct_module/commands/*.py
• README.md, docs/cli-reference.md

Acceptance criteria

• New commands are available with help text and examples.
• Output mode names are consistent in CLI, MCP, and docs.
• Tests for output and completion installer.