Sticky comment for review mode

Hi there!

We are currently using a [forked version](https://github.com/Uniswap/claude-code-action/tree/feat/sticky-comment) of the action to enable sticky comments in review mode. I'm happy to create a PR for this, but wanted to confirm/explore what it could look like. (we would prefer to not maintain a fork)

We want to continue to use the sticky comment in review mode for a more thorough analysis/overview of the PR, and we only want one comment because we use stacked diffs, and often rebase/force push to a given pr many times. If the bot were to use the comment functionality each time and put the whole thing in the body of the review, it will clog up the PR significantly.

the changes I've done at a high level:
- sticky comment works only with a custom override prompt
- make sure we persist content of comment when the action runs again (so the action can learn from what was already posted)
- it injects a (configurable) hidden comment so we can find it between runs

cc @ashwin-ant