get_file_contents fetch refs improvements

[almaleksia]

Summary

This PR improves ref resolution handling in get_file_contents tool.

1. Sometimes LLM passes sha in ref parameter which results in failure. This PR autocorrects it allowing shas to be passed as refs.
2. Return custom errors when tool tried to resolve main branch and failed - suggest to try master instead.

Why

We have lots of errors like failed to resolve git reference: could not resolve ref "50e4e8b9178ca5c4e3c60d0022b3b041cc58c188" as a branch or a tag

Which means sha is passed as ref, this change aims to decrease amounts of such failures.

What changed

MCP impact

• No tool or API changes
• Tool schema or behavior changed
• New tool added

Prompts tested (tool changes only)

Security / limits

• No security or limits impact
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

Docs

• Not needed
• Updated (README / docs / examples)

[Copilot]

Pull request overview

This PR enhances the get_file_contents tool's ref resolution by auto-detecting Git commit SHAs passed in the ref parameter and providing clearer error messages when the 'main' branch is not found.

• Adds looksLikeSHA() function to detect full 40-character commit SHAs and bypass branch/tag resolution
• Implements custom error messages suggesting 'master' when 'main' branch is not found
• Adds comprehensive test coverage for the SHA detection logic

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

File	Description
pkg/github/repositories.go	Adds looksLikeSHA() helper function and integrates SHA detection into resolveGitReference(); adds custom error messages for missing 'main' branch
pkg/github/repositories_test.go	Adds unit tests for looksLikeSHA() function and integration tests for SHA-based ref resolution in Test_resolveGitReference()

[mattdholloway]

looks great! looksLikeSHA is a really good idea

[SamMorrowDrums]

Very nice. Out of interest, do we have a way of looking up the default ref? And should we just try master or versa and just say what we did as part of the response? I guess it's just how things are that we need heuristics like this. But higher % success of achieving actual intent feels reasonable.