feat: add Edit on GitHub link to documentation pages

[barredterra]

No description provided.

[greptile-apps]

Confidence Score: 4/5

The PR is not yet safe to merge because stale remote-tracking metadata can expose an Edit on GitHub link that targets a deleted branch.

Branch selection validates cached local Git metadata rather than the branch's current existence in the configured GitHub repository, so a deleted branch can still be embedded in the edit URL.

Files Needing Attention: compendium/docs.py

Prompt To Fix All With AI

### Issue 1
compendium/docs.py:612
**Stale remote ref breaks link**

If a canonical remote branch is deleted after the checkout last fetched it, the locally cached remote-tracking ref still passes this check, causing the displayed Edit on GitHub link to target the deleted branch and open a 404.

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For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (4): Last reviewed commit: "fix: resolve GitHub edit branch from the..." | Re-trigger Greptile}

[0xD0M1M0]

What about Private Repos?

[barredterra]

What about Private Repos?

Users will be told by GitHub that they don't have edit access.