gc rig add should accept --prefix

[corymhall]

Summary

gc rig add should support an explicit --prefix flag so the rig prefix can be chosen at creation time.

Right now gc rig add accepts the rig path, --include, and --start-suspended, but not a prefix override.

Problem

The rig prefix is an important part of the workspace identity because it affects beads IDs and how rigs are distinguished in practice.

When adding a rig, there are cases where the automatically chosen/default prefix is not what the user wants. Today that means the user has to manually edit config after the fact instead of setting the desired prefix up front.

That is awkward for a command whose job is to register a rig cleanly in one step.

Proposed behavior

Allow:

gc rig add /path/to/project --prefix myrig

This should write the provided prefix into the new rig entry in city.toml.

Why this would help

• makes gc rig add more complete as a rig-registration workflow
• avoids immediate manual edits after adding a rig
• helps users choose stable, readable bead prefixes intentionally
• makes scripted setup easier and less error-prone

Notes

Current gc rig add --help shows support for:

• path
• --include
• --start-suspended

but no way to set a prefix directly.

Open questions

• should --prefix be required only when inference would be ambiguous, or always optional?
• should gc rig add validate uniqueness of the prefix before writing config?
• if omitted, should current default/inference behavior remain unchanged?

[julianknutsen]

Closed by #339