Implement depth option for multi-repo discovery

[rsanheim]

Implement --scan-depth to control how deep we search for git repositories. Currently hardcoded to depth 1 (immediate subdirectories only).

Spec (from docs/SPEC.md §2.1 & §6.4)

• Default search depth MUST be 1 (immediate subdirectories only).
• When --scan-depth N is specified, search up to N levels deep.
• When --scan-depth all is specified, search recursively without limit, stopping at .git boundaries.
• Option MUST accept a positive integer or "all"; default is 1.
• --scan-depth is the only discovery-depth option; --depth MUST NOT be accepted for discovery.

Notes

--scan-depth is the only supported discovery-depth flag to avoid conflicts with git's own --depth.

References

• docs/SPEC.md §2.1 Discovery Algorithm: https://github.com/rsanheim/git-all/blob/main/docs/SPEC.md#21-discovery-algorithm
• docs/SPEC.md §6.4 --scan-depth: https://github.com/rsanheim/git-all/blob/main/docs/SPEC.md#64---scan-depth
• docs/index.md (CLI overview): https://github.com/rsanheim/git-all/blob/main/docs/index.md
• docs/dev/git-all-future.md: https://github.com/rsanheim/git-all/blob/main/docs/dev/git-all-future.md

[rsanheim]

Rust implementation done in PR #20. Zig/Crystal TBD.