Feature request: Offline mode

### Describe the feature or problem you’d like to solve

One of the benefits of `git` is that developers do not need to be connected to the internet continuously in order to interact with the VCS system. It would be useful if this functionality were extended to things like issues and pull requests.

### Proposed solution

The essential functionality would consist of a pair of commands `offline`/`online`. Running `offline` would make a local cache of issues and PRs, as well as fetch commits associated with PRs. Commands run after `offline` and before `online` would insert API calls into a local queue (probably doable with a simple JSON file on disk). Running `online` would then loop through this queue and execute all the actions performed in "offline" mode.

### Additional context

This is definitely a major feature -- I'd be interested in working on it, but only if there's a decent chance of a PR being merged.

The issue names no files, tests, or entry points. Start by surveying the existing issue and pull-request commands and their GitHub API interactions, then define the cache, queued actions, and commit-fetching scope. Done means offline and online commands support the agreed issue and PR operations reliably.

Written by the indexing model from the issue text.