feat(KB-201): add settings CLI commands

- Add 'kb settings' command with get, set, list, and unset subcommands
- Create settings.ts command module with full type validation
- Add comprehensive unit tests for settings commands (432 lines)
- Wire settings routing into CLI entry point (bin.ts)
- Add changeset for patch release of @dustinbyrne/kb package

.changeset/add-settings-cli-commands.md

@@ -0,0 +1,5 @@
+---
+"@dustinbyrne/kb": minor
+---
+
+Add CLI commands for settings management: `kb settings` and `kb settings set <key> <value>`.