feat: bmalph config — platform switching without reset

[LarsCowe]

Summary

Currently, changing the platform after bmalph init requires bmalph reset (which destroys all state) or manual editing of bmalph/config.json. Neither is ideal.

Proposed Solution

A bmalph config command (or bmalph config set platform <id>) that:

1. Validates the new platform ID
2. Updates bmalph/config.json with the new platform
3. Swaps the instructions file snippet (removes old platform's snippet, writes new one)
4. Handles command delivery migration:
   • directory → none: warns about orphaned command files
   • none → directory: creates command directory and copies commands
   • inline → none: removes inline commands section from instructions file
5. Updates _bmad/config.yaml platform field
6. Updates .ralph/.ralphrc PLATFORM_DRIVER value

Options

• --dry-run — Preview changes without writing
• bmalph config get <key> — Read a config value
• bmalph config set <key> <value> — Set a config value
• bmalph config list — Show all config values

Risks

• Command delivery migration is the tricky part — going from directory-based (Claude Code) to none (Cursor) leaves orphaned .claude/commands/ files
• Instructions file manipulation needs to handle partial/corrupted content gracefully

Additional Context

This would significantly improve the developer experience for users who want to try different platforms without losing their BMAD planning artifacts and Ralph state.