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

[OllieinCanada]

Sure. The use case is a local repository where the requested review starts with the repository's initial (root) commit—for example, a small reproducer repository, a newly initialized repository, or imported history whose first commit also needs review. Today, when no baseline is supplied, sashiko-cli constructs ^ as the default baseline. A root commit has no parent, so Git cannot resolve that revision and the review exits before analyzing the commit. This PR handles only that boundary case by reviewing the root commit's tree directly. Commits that have a parent continue through the existing parent-baseline and patch-application path unchanged. The failure shown in #203 is consistent with this root-commit case, though confirmation from the reporter that the target SHA is the repository's root commit would remove the remaining uncertainty.