docs: design gr2 repo maintenance and lanes

[laynepenney]

Summary

• add a design doc for explicit gr2 repo maintenance and named multi-repo lanes
• add a small prototype planner that separates structural apply from repo sync policy
• anchor the follow-on implementation sequence for repo status, lane metadata, and lane commands

Why

gr gave us the right lessons, but it still mixes global sync behavior with active feature work. This design moves gr2 toward:

• narrow structural apply
• explicit repo-maintenance surfaces
• cheap named multi-repo scratchpads for humans and agents

Closes #542.

Included

• docs/PLAN-gr2-repo-maintenance-and-lanes.md
• gr2/prototypes/repo_maintenance_prototype.py
• gr2/prototypes/README.md

Verification

• python3 -m py_compile gr2/prototypes/repo_maintenance_prototype.py
• disposable workspace demos for path-conflict and dirty/autostash classification

Next steps

1. implement gr2 repo status
2. define lane metadata on disk
3. add gr2 lane create/status/enter/remove
4. add gr2 lane checkout-pr

[laynepenney]

No blocker from final pass. The PR now contains the full design package: lane/repo-maintenance design, rulebook, current-surface audit, and two prototypes that make the next implementation slices concrete.