Other sharing options

### Feature summary

Add more sharing options beyond secret gists

### What problem are you trying to solve?

Can we have some other sharing options besides secret gists? My account is an enterprise managed account and we are not allowed to create gists of any kind, but there are many times I will want to share a session with a coworker. I would like some other way to share a conversation rather than trying to copy the contents and then edit the formatting.

### Proposed solution

An option to copy a markdown-formatted conversation, or a plaintext conversation, or a way to save a session to a file. I know the CLI has several more sharing options but the app version only has gists and /share doesn't work.

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

_No response_

Start by reviewing the app's current gist-sharing flow and comparing it with the CLI's existing sharing options. Determine which non-gist option—Markdown, plaintext, or saving a session to a file—is in scope, then verify that the chosen workflow supports enterprise-managed accounts and avoids manual copy-editing.

Written by the indexing model from the issue text.