Need per-repo checkout/sync for single-repo workflows

[laynepenney]

Problem:\n- When working on a single repo while other repos are on different branches, gr checkout/sync/rebase/merge operate across all repos.\n\nImpact:\n- Forces raw git usage for routine single-repo work and conflict resolution.\n\nExpected:\n- Add --repo (and/or --group) to target a single repo for checkout, sync, rebase, and merge without touching others.\n- Non-target repos should be skipped with a clear warning.

[laynepenney]

Had to use raw git in the gitgrip repo to cut a clean branch off origin/main (per-repo checkout/rebase isn’t possible with gr today). Tracking here.

[laynepenney]

Friction: cleaning up merged PR branches requires per-repo checkout/delete, but gr checkout/gr branch -d can’t target a single repo. Had to use raw git for gitgrip + manifest to switch back to codi-gripspace and delete local branches.

[laynepenney]

Used raw git again to switch and delete a manifest branch (griptree PR branch cleanup) because gr checkout/branch -d can’t target a single repo.