refactor(cli): restructure commands from flat to nested subcommands
Change CLI structure from `brewlog {verb}-{entity}` to `brewlog {entity} {verb}`:
- brewlog add-roaster → brewlog roaster add
- brewlog list-bags → brewlog bag list
- brewlog create-token → brewlog token create
- etc.
Each entity module now owns its subcommand enum and dispatch logic,
simplifying main.rs and improving discoverability via `brewlog {entity} --help`.
- Add RoasterCommands, RoastCommands, BagCommands, GearCommands, TokenCommands enums
- Add run() dispatcher to each entity module
- Simplify top-level Commands enum to delegate to entity modules
- Update all CLI tests and bootstrap script
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