#11115: [AI-assisted] Add /model-set chat command with autocompletion and search/pick option

This PR was assisted by OpenCode. Testing status: lightly tested in local sessions. This pull request introduces a new `/model-set` chat command that allows users to search for and pick the active model in a session, along with supporting infrastructure for model selection and user feedback. It also updates help and UI text throughout the codebase to reference the new command and exposes model picker catalog utilities for broader use. [Watch the demo video](https://1drv.ms/v/c/09ad408b39291cf7/IQAINAYkPHfdSbJl91OSCW9gAYM29NVcn45tPL0fTUUs9IQ?e=8SmGqh) And here is some screenshot: <img width="416" height="572" alt="image" src="https://github.com/user-attachments/assets/96d22d5b-acc2-4430-abee-cf60d30ea6e0" /> <img width="325" height="260" alt="image" src="https://github.com/user-attachments/assets/442f03eb-3712-4ed9-a059-3f6e5bf9fb7d" /> <img width="521" height="103" alt="image" src="https://github.com/user-attachments/assets/c205ff1d-a04f-464c-bbc2-b04c9316125f" /> <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR adds a new `/model-set` command to search/pick the active model, wires it into the command registry/dispatcher, and updates model directive help text and the trigger-handling E2E test expectations to mention the new command. It also exports `buildModelPickerCatalog` so the new command can reuse the model picker catalog logic for autocompletion choices. <h3>Confidence Score: 4/5</h3> - This PR is close to safe to merge, with a couple of issues to address first. - Core wiring for the new command is straightforward and updates appear consistent across registry/handler/help text, but there is a clear formatting issue in an E2E test and the new command currently forces an uncached model-catalog load on every invocation, which is likely to be a real performance/regression concern in interactive use. - src/auto-reply/reply/commands-model-set.ts; src/auto-reply/reply.triggers.trigger-handling.shows-quick-model-picker-grouped-by-model.e2e.test.ts <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->