feat: add base-ref input to override PR base branch

[jimhigson]

Closes #145.

Summary

• Adds a base-ref action input that overrides the PR base branch when resolving the comparison ref
• When set, baseSha is nulled so fallback logic doesn't re-use the PR's base.sha
• Null guards on the fetch and reset fallbacks surface a clear error instead of running git ... null
• README example shows a release-please conditional: default PR base for feature PRs, production for release PRs

[jimhigson]

example of this working as planned here: jimhigson/head-over-heels-online#773 (comment)

[rschristian]

Sorry for the delay, needed a bit of a break from GitHub tbh.

Just a few minor comments, I'll apply them directly & merge in to not require more of your time. We really appreciate it! Will try to cut a release in the couple of days to make this available.