feat: add gr2 lane metadata surface

[laynepenney]

Summary

• add a first read-write gr2 lane surface with create/list/show/remove commands
• persist lane metadata under .grip/state/lanes and scaffold lane roots under agents//lanes/
• validate repo membership and record lane context, exec defaults, PR associations, and recovery state

Closes #547

Testing

• cargo check -p gr2-cli --quiet
• cargo test --test cli_tests --quiet

[laynepenney]

First implementation slice for lane metadata is up.

What this covers:

• gr2 lane create/list/show/remove
• persisted lane records under .grip/state/lanes//.toml
• lane root scaffolding under agents//lanes/
• validation of repo membership, branch maps, PR associations, and default context roots

Local verification:

• cargo check -p gr2-cli --quiet
• cargo test --test cli_tests --quiet