[Feat] Add Targets

[bean1352]

What

Adds named targets to cli.yaml, so one project directory can point at several Cloud instances, for example staging and production.

• powersync link cloud --target=<name> --instance-id=<id> stores a link under targets.<name>. The top-level fields stay the default. --create --target=<name> creates the instance and links it in one step.
• Pick a target per command with --target=<name> or POWERSYNC_TARGET. --instance-id still overrides both.
• The target instance banner from [Feat] Show target instance before deploy, stop, destroy and compact #64 shows the selected target name.
• powersync fetch instances lists the targets of each linked directory.

Backwards compatible

A cli.yaml without a targets key works exactly as before. Older CLI versions ignore the key and POWERSYNC_TARGET and use the top-level fields.

Small fixes

• Commands that accept Cloud and self-hosted instances let --instance-id or --api-url decide the context even when cli.yaml links the other type.
• A linked directory without service.yaml now suggests powersync pull instance.
• Invalid IDs in cli.yaml are reported with the field name, for example targets.staging.instance_id in cli.yaml, instead of a flag name.

AI disclosure

I used Claude to research the codebase and implement the change. I reviewed and tested it myself.

[LucDeCaf]

This looks good to me :) Just some minor comments.

[benitav]

I understand that --environment is the most common use case for multiple instances, but since "environment" doesn't actually mean anything here, and customers can have multiple instances in the same environment, I think we should call this flag --target or similar to make clearer that you're targeting a specific instance, not environment, with this. --alias could be another option.

[bean1352]

I understand that --environment is the most common use case for multiple instances, but since "environment" doesn't actually mean anything here, and customers can have multiple instances in the same environment, I think we should call this flag --target or similar to make clearer that you're targeting a specific instance, not environment, with this. --alias could be another option.

I've opted to rename the variable to target.