#8210: ui: show subagent labels in chat session dropdown

Shows labeled subagent sessions in the chat session dropdown by appending the session key for subagent entries, making it easier to identify what each run is. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Updates the session dropdown display-name resolution so subagent sessions are easier to distinguish by appending the session key to the displayed name. This is done in `ui/src/ui/app-render.helpers.ts` by detecting subagent session keys (`:subagent:`) and including the key in the option label when a `displayName` is present, while preserving existing labeling behavior. <h3>Confidence Score: 4/5</h3> - This PR is likely safe to merge, with a minor UX/consistency concern around when session keys are appended. - Change is small and localized to display-name formatting for session dropdown options; no state or API behavior changes. The main risk is inconsistent UI output between `label` and `displayName` paths, which may not match the stated intent. - ui/src/ui/app-render.helpers.ts <!-- greptile_other_comments_section --> _{(4/5) You can add custom instructions or style guidelines for the agent [here](https://app.greptile.com/review/github)!} <!-- /greptile_comment -->