fix(editor): resolve getTree cache staleness by using commit SHA instead of branch name

[devin-ai-integration]

Fixes stale cache issue when fetching docs.yml content for the Editor.

Short description of the changes made

Modified getTree in github-loader.ts to resolve the branch ref to a commit SHA before caching. Previously, the tree was cached using the branch name (e.g., "main") as the cache key with no revalidation period, causing the cache to potentially live forever even as the branch moved forward.

Key changes:

• Resolve ref to commit SHA via resolveRefToSha before making the GitHub API call
• Use commit SHA in both the API request and cache key (making the cache immutable per commit)
• Add github-repo-${owner}-${repo} tag to enable cache invalidation via invalidateRepoCache
• Integrated with existing skipCache functionality from app branch

What was the motivation & context behind this PR?

The Editor was showing stale docs.yml content when creating new sessions. The root cause was that getTree cached the repository tree based on branch name with no revalidate period, so once cached, the tree data would never refresh even when new commits were pushed to the branch.

Now when the branch moves to a new commit, getCommitRef (with 5-min TTL) will return the new SHA, resulting in a new cache key for getTree and fresh tree data.

How has this PR been tested?

• Code analysis and review of the caching flow
• Lint checks passed
• Dashboard dev server compiles and starts successfully

Human review checklist

• Verify the error handling is appropriate - the function now throws if ref resolution fails (previously would have attempted the API call with the branch name)
• Confirm the additional resolveRefToSha call overhead is acceptable (it's cached with 5-min TTL)
• Consider if the 5-minute TTL on getCommitRef is acceptable for New Session freshness requirements
• Verify skipCache functionality still works correctly with the new commit SHA-based caching

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Link to Devin run: https://app.devin.ai/sessions/66976b7aab5a4c25b2b2278dd58217a2
Requested by: stephen@buildwithfern.com

[devin-ai-integration]

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