Remember max mode setting per-thread and add a user setting (#30042)

Supersedes: https://github.com/zed-industries/zed/pull/29936

Thanks for your contribution @imumesh18, but we had a slightly different
take on it :)

Release Notes:

- N/A

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

assets/settings/default.json

@@ -646,6 +646,8 @@
     "version": "2",
     // Whether the agent is enabled.
     "enabled": true,
+    /// What completion mode to start new threads in, if available. Can be 'normal' or 'max'.
+    "preferred_completion_mode": "normal",
     // Whether to show the agent panel button in the status bar.
     "button": true,
     // Where to dock the agent panel. Can be 'left', 'right' or 'bottom'.