Improve preview link discovery

[skylarkning]

Summary

• The preview-link bot now will follow referenced PRs as well as issues.
• Let it reuse paths from existing Main and Deploy preview links.
• Keep broken share.firefox.dev links non-fatal so the bot can keep looking or fall back to /.

Example

If PR #6137 says Follow-up to #6017, and #6017 has:

[Main](https://main--perf-html.netlify.app/public/abc/)

then the bot can add this to PR #6137:

[Main](https://main--perf-html.netlify.app/public/abc/) | [Deploy preview](https://deploy-preview-6137--perf-html.netlify.app/public/abc/)

Test

node --test .github/scripts/add-preview-links.test.mjs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.50%. Comparing base (b894055) to head (b2c0eba).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6149   +/-   ##
=======================================
  Coverage   83.50%   83.50%           
=======================================
  Files         342      342           
  Lines       36522    36522           
  Branches    10221    10224    +3     
=======================================
  Hits        30497    30497           
  Misses       5597     5597           
  Partials      428      428           

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