feat: implement update checking

[lusu007]

Pull Request

Description

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• ♻️ Refactoring
• 📚 Documentation
• 🔧 Chore

Related Issues

None

Testing

• Unit tests pass
• Manual testing completed
• New tests added for new functionality

Checklist

• I have performed a self-review of my own code
• Tests have been added/updated and pass (if needed)
• Documentation has been updated (if needed)

[Copilot]

Pull request overview

Adds CLI release update checking so grounds version and grounds doctor can surface newer releases and, when possible, suggest how to upgrade. This fits the CLI’s diagnostics/versioning surface by extending the existing version and doctor commands.

Changes:

• Added release lookup, version comparison, and install-method detection under internal/version.
• Extended grounds version and grounds doctor to report update status and richer diagnostic output.
• Updated README command docs and added unit tests for the new version/doctor behavior.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
README.md	Documents version --check and doctor’s update-warning behavior.
internal/version/install_method.go	Adds install source detection and upgrade command hints.
internal/version/install_method_test.go	Adds unit tests for install-method detection.
internal/version/check.go	Adds latest-release lookup and version comparison logic.
internal/version/check_test.go	Adds tests for release checking and version comparison.
cmd/grounds/commands/version.go	Extends grounds version with optional update checks and hints.
cmd/grounds/commands/version_test.go	Adds command-level coverage for version --check.
cmd/grounds/commands/doctor.go	Adds version checking and new structured doctor output/spinner flow.
cmd/grounds/commands/doctor_test.go	Adds tests for the new doctor rendering and version-check behavior.

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