Replace obsolete toit.pkg commands

[floitsch]

Summary

• replace every toit.pkg invocation and prose reference with toit pkg
• update snippet CI to use the package subcommand on the toit executable
• update the local-package example to use uninstall and --prefix
• remove the obsolete space-versus-dot guidance

Validation

• verified no toit.pkg references remain in tracked source
• verified current install/uninstall command syntax
• ran the affected package-documentation snippet checks
• ran git diff --check