fix: reject a remote-tracking --base up front instead of failing at PR creation

[vitali87]

Summary

pr-split split feature --base origin/main passed input validation (git rev-parse --verify origin/main succeeds), so the whole split ran — branches created and pushed, plan saved with base_branch="origin/main" — and only failed at the very end when gh pr create --base origin/main was rejected because GitHub has no branch of that name. The same applied to a tag or a SHA, and to execute on such a plan.

A new _require_local_branch(base) refuses anything that is not refs/heads/<base> before any worktree, prompt or branch creation, in both split and execute:

Base 'origin/main' is not a local branch; sub-PRs are opened against it on GitHub, so pass the branch name (for example 'main')

The "for example" hint is only added when the input is <remote>/<name> (or refs/remotes/<remote>/<name>) for a remote listed by git remote; tags and SHAs get the message without a bogus suggestion.

Test plan

• TestValidateInputs: origin/main rejected with the hint, tag/SHA/feature/topic rejected without one, refs/remotes/origin/main suggests main, local base checks the branch head; execute refuses an old plan with a remote-tracking base before the confirm prompt — all fail on main
• Review verified on a real repo with a bare origin (split and execute paths); a local branch literally named origin/main is still accepted
• 449 tests pass, ruff clean
• Local review: 5/5

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📥 Commits

Reviewing files that changed from the base of the PR and between ec2f754 and 94613fd.

📒 Files selected for processing (3)

• pr_split/cli.py
• pr_split/exceptions.py
• tests/test_cli_coverage.py

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