Add --cheatsheet subcommand

[Bortlesboat]

Add just --cheatsheet to print a quick-reference guide, following the same include_str! pattern as --changelog. Closes #3108.

• CHEATSHEET.md at repo root, embedded at compile time
• Covers recipes, variables, dependencies, attributes, settings, modules, built-ins, and CLI flags
• Test follows the changelog test pattern

[Bortlesboat]

Saw #3108 — embedding it in the binary as just --cheatsheet makes a lot of sense, consistent with --man and --changelog. I'll rework this PR to add it as a subcommand instead of a standalone file.

[Bortlesboat]

Closing — superseded by #3108.