fixes validation issue where it was honly comparing base to itself, n…

[funkyshu]

…ot the staged changes

[Copilot]

Pull request overview

This PR fixes releasegen validate --require-changelog-entry so it correctly detects local pre-commit changes (where HEAD hasn’t advanced yet) and compares changelog content against what will actually be committed (the git index), rather than relying on the working tree / HEAD tree alone.

Changes:

• Extend VCS diffing to include worktree status in ChangedFiles, enabling pre-commit scenarios where changes are staged but not committed.
• Add FileAtIndex and update validation to compare [Unreleased] against the staged (index) version of the changelog.
• Add/extend tests covering staged-but-uncommitted changes and staged-vs-unstaged changelog behavior; document the behavior in the project changelog.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
internal/vcs/git.go	Adds worktree-aware changed-file detection and introduces FileAtIndex for reading staged content.
internal/vcs/git_test.go	Adds coverage for pre-commit changed-file detection and FileAtIndex behavior across staged/unstaged/untracked cases.
cmd/releasegen/validate.go	Switches changelog comparison to staged/index content to match what the next commit will contain.
cmd/releasegen/validate_test.go	Adds regression tests for pre-commit detection and for rejecting unstaged changelog edits.
CHANGELOG.md	Documents the validation behavior changes (worktree-inclusive diffs and index-based changelog comparison).

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