Update changelog for v6.0.3

[yaananth]

Updates CHANGELOG.md for the v6.0.3 release.

Includes:

• SHA-256 repository checkout initialization support (Fix checkout init for SHA-256 repositories #2439)
• SHA-256 merge commit matching/test coverage (fix: expand merge commit SHA regex and add SHA-256 test cases #2414)

[Copilot]

Pull request overview

Updates the project changelog to document the v6.0.3 release, capturing the two SHA-256-related fixes that were recently merged.

Changes:

• Add a new ## v6.0.3 section at the top of CHANGELOG.md.
• Record the two included PRs: SHA-256 repo init support (#2439) and SHA-256 merge commit SHA/test coverage updates (#2414).

Show a summary per file

File	Description
CHANGELOG.md	Adds the v6.0.3 release notes with links to the two included pull requests.

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• Files reviewed: 1/1 changed files
• Comments generated: 0

[dimanardiyansah123-stack]

Bl@

[dimanardiyansah123-stack]

• BIH