fix: install auggie commands to augment directory

[dylanbuchi]

What

• Update the Auggie installer defaults and docs so CLI installs now target .augment/commands for both workspace and
  user scopes.

Why

• Auggie’s Custom Slash Commands guidance expects commands under .augment/commands, so the prior .auggie paths were writing to a deprecated location (https://docs.augmentcode.com/cli/custom-commands).

How

• Swapped Auggie installer defaults/detection to .augment/commands, ensuring project-relative paths resolve correctly.
• Adjusted pre-collected location handling so .augment/commands maps to the project directory before installation.
• Synced the Auggie CLI docs to reference .augment/commands for home and workspace setups.

Testing

• npm run lint

[alexeyv]

I can't test this (not an Auggie CLI user), but this looks like a simple change that matches their documentation.