feat: add workmux command for generating workmux configuration

[sysid]

Summary

• Add new rplc workmux command that generates a .workmux.yaml configuration file for workmux integration
• The generated config includes post_create hooks for direnv setup and files.symlink entries for all rplc-managed files
• Add ConfigParser.add_config_entry() helper method for appending entries to the rplc-config section

Details

The workmux command:

• Requires swapped-in state (at least one sentinel must exist)
• Generates .workmux.yaml with direnv hooks (ln -s $SOPS_PATH/dot.envrc .envrc and direnv allow)
• Lists all rplc-managed files in the symlink section, including .workmux.yaml itself
• Auto-adds .workmux.yaml to the rplc config file if not already present
• Inserts config entries directly after the last content line (no blank lines)

Test plan

• Test workmux fails when not swapped-in
• Test workmux generates correct YAML content
• Test workmux adds entry to rplc config
• Test no duplicate config entries are added
• Test add_config_entry handles various edge cases
• All 122 tests pass