Validate branch mutation preflights

[Raithlin]

Summary

• Validate branch create, rename, and delete before returning a dry-run plan or making a change.
• Reject unsafe or unverifiable origin operations before local mutation.

Changes

• Check branch names, start commits, local and origin ref collisions, worktree use, merge state, and provider safety.
• Read origin refs from Git's effective push target and match Git's upstream rule for local deletion.
• Update command help, capabilities, and the command guide; move the completed dry-run item out of the roadmap priorities.

Validation

• GOCACHE=/tmp/gha-review-fix-gocache make check — passed lint, race tests, and the 95.0% coverage gate.
• Focused branch and safety tests passed in internal/git, internal/branch, and internal/commands.
• git diff --check passed.

Checklist

• Tests cover the changed behavior.
• Documentation and command help are updated where needed.
• Structured output contracts remain backward-compatible, or the compatibility decision is documented.
• Mutating workflows disclose dry-run and confirmation behavior. Existing branch mutations still execute by default; this PR adds preflight validation and does not change their confirmation model.