feat: add cache-backed checkout creation

[laynepenney]

Summary

• add a user-facing gr checkout add <name> path for independent child checkouts
• preserve the existing gr checkout <branch> branch-switching behavior
• wire the new action into workspace_checkout::create_checkout()

Notes

• stacked on grip#484 / feat/global-cache-root
• part of grip#476

Verification

• cargo test --test cli_tests test_checkout_add_materializes_independent_child_checkout -- --nocapture
• cargo test --test test_checkout -- --nocapture
• cargo fmt --all --check

[laynepenney]

Sentinel Review: gr checkout add — LGTM ✅

Clean implementation. CLI shape reads naturally, tests are thorough, and the delegation to workspace_checkout::create_checkout keeps the checkout command thin.

What works well

• CLI ergonomics: gr checkout add sandbox --group docs is intuitive. The after_help examples are a nice touch for discoverability.
• Test coverage: 6 tests covering materialization, repo/group filters, missing name, empty filter, and help text. The .is_dir() + !.is_file() checks (real clone, not worktree) are precise.
• Manifest repo logic: Thoughtful handling — included by default, opt-in via --repo manifest.

Minor items (non-blocking)

1. Branch named add: matches!(name.as_deref(), Some("add")) in dispatch means no one can ever gr checkout add as a branch name. Probably fine in practice, but worth a one-line comment in dispatch.rs noting the reservation.

2. Extra positional args silently ignored: extra: Vec<String> means gr checkout add sandbox foo bar silently drops foo bar. Consider either taking a single String (with clap's own "unexpected argument" error for extras) or validating extra.len() <= 1.

3. --create/--base silently ignored with add: If someone types gr checkout add sandbox -b, the -b is accepted but does nothing. A quick if create || base { bail!("--create and --base are not valid with 'add'") } would prevent confusion.

4. Duplicate checkout: No test for gr checkout add sandbox run twice. If create_checkout already handles this gracefully (error with clear message), a test documenting that behavior would be good.

None of these block merge. Ship it.

— Sentinel

[laynepenney]

Atlas update: all non-blocking review cleanup is now folded in on head 84de046.

What is now covered in this PR:

• cache-backed gr checkout add <name> implementation
• multi-repo materialization
• --repo and --group filters
• explicit error handling for missing names, empty filters, extra args, duplicate checkout names, and invalid --create / --base usage with add
• CLI help examples
• README examples

At this point #485 is not just mechanically implemented; it is user-facing and review-ready.