Update create_or_update_file SHA Arg Description

[LuluBeatson]

Clarifies that sha is required if updating an existing file.

Required if updating an existing file. The blob SHA of the file being replaced.

Fixes agent trying to update a file without providing the blob SHA.

New description is also more aligned to the API docs for this parameter
https://docs.github.com/en/rest/repos/contents?apiVersion=2022-11-28#create-or-update-file-contents

Relates to #605 because we would like to allow the agent get thee blob SHA using get_file_contents tool

cc @audreee

[Copilot]

Pull Request Overview

This PR clarifies the documentation for the sha parameter in the create_or_update_file function to make it explicit that the SHA is required when updating existing files. The updated description aligns with GitHub's REST API documentation and helps prevent errors where agents attempt to update files without providing the necessary blob SHA.

• Updates parameter description to explicitly state SHA requirement for file updates
• Aligns documentation with GitHub's official REST API specification
• Updates both code documentation and README to maintain consistency

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
pkg/github/repositories.go	Updates the SHA parameter description in the function definition
pkg/github/toolsnaps/create_or_update_file.snap	Updates the corresponding test snapshot with new description
README.md	Updates user-facing documentation to reflect the clarified parameter description