cli: support local review of root commits

[OllieinCanada]

Fixes #203

Summary

Allow sashiko-cli local to review a repository's root commit.

Root commits are checked out and reviewed directly because they have no parent
baseline. Ordinary commits continue through the existing parent-based patch
application path.

Root cause

The local CLI always constructed <first commit>^ when no baseline was given.
That revision is invalid when the requested commit is the repository's first
commit, so the review subprocess exited before producing a result.

Changes

• inspect the first commit's parent list before selecting the default baseline
• use the direct-commit path for a parentless root commit
• preserve root-commit file discovery and commit-message context for review
• avoid constructing an empty root..root series range
• add end-to-end CLI coverage for root and ordinary commits

Validation

• reproduced the failure with the real CLI and review subprocess
• root-commit and ordinary-commit CLI regression tests pass
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features --release -- -D warnings
• ordinary release suite passes with the unrelated issue tests: make bad-tag recovery test portable across Git versions #350 portability test
  excluded
• DCO sign-off check passes

No external Git server, AI provider, or paid API is required by the regression
tests.

[derekbarbosa]

Hello,

Allow sashiko-cli local to review a repository's root commit.

Root commits are checked out and reviewed directly because they have no parent
baseline. Ordinary commits continue through the existing parent-based patch
application path.

would you mind elaborating on this? I am having a bit of trouble understanding the perceived usecase here