feat: add an interactive config editing command

[nonz250]

Summary

Add a command to edit config.yml interactively, either one repository at a time or by walking through every repository in turn.

Problem

After init, the only way to change a setting is to hand-edit config.yml. add exists but only appends a repository, and it rewrites the file through YAML.stringify, which discards comments and formatting.

A real config has nine repository entries, each carrying a full label block, so the file is long and repetitive. Changing one field - auto_impl_after_plan, or adding the missing default_branch - means locating the right block by hand and getting the indentation right.

Scope

• New command (for example sabori-flow edit).
• Pick a single repository to edit, or walk all repositories one by one.
• Edit execution.* and language as well, not only repositories.
• Offer the current value as the default for every prompt so that pressing Enter is a no-op.
• Allow removing a repository.
• Preserve unknown keys, and preserve comments and formatting where practical.
• State when a change requires reinstall (interval_minutes is the one that does).

Considerations

• add should reuse the same per-repository prompt flow (src/commands/helpers/repository-prompt.ts), so the two commands stay consistent.
• Should follow feat: add an interactive config migration command #113, which settles how config.yml is written back (surgical edit vs. full reformat) and introduces the schema version.