Use functional workspace config

Forest stores registered checkout metadata in .forest/config.yaml, not in a generated summary file. Commands use this file as source of truth for checkout registration and validation, while .forest/HEAD remains the user-local active checkout pointer.

Consequences

• checkout create and checkout adopt update .forest/config.yaml.
• checkout list reads .forest/config.yaml.
• Checkout directories not listed in .forest/config.yaml are repair/adopt candidates, not active registered checkouts.