feat(projects): apply the global model default to all existing projects

[coygeek]

Summary

Add a global control in Settings → Projects that makes all existing projects use the current default model for new threads, including projects that still have an older model override. Users with many project folders should be able to switch their default model without updating every folder individually.

Problem to solve

I have accumulated many project folders while using GPT 5.6 Sol. After switching to GPT 6.0 Astra, I changed the model under Settings → Projects → All machines → All projects. However, my existing projects still retain GPT 5.6 Sol as their project override.

The settings screenshots show the mismatch: All projects is set to GPT 6.0 Astra with High reasoning, while an existing project is marked “Overridden” and remains on GPT 5.6 Sol with High reasoning. The project settings say “Overridden for this project. Reset to use the default model.”

When I start a new thread in one of these older projects, it still defaults to the old model. If I miss the model selector before sending, I end up using a model I intended to move away from. Resetting each project individually becomes tedious with many folders, and the same cleanup is needed whenever I change my preferred model.

Proposed behavior

Provide an explicit global action, such as “Apply default to all existing projects,” that lets me make existing projects follow the current global model default even when they already have a project override. After applying it, future changes to the global default should carry through to those projects without repeating the cleanup.

Make it clear that this action replaces existing project model overrides and which machines and projects it covers. Keep the existing connected-machine scope clear: the current settings screen says offline machines keep their defaults, so the action must not imply those machines were updated if they were skipped.

Changing the default alone can continue to preserve project-specific choices. The missing capability is an explicit way to apply my global preference across existing projects in one operation.

Acceptance criteria

• Given multiple existing projects with GPT 5.6 Sol overrides, when the global default is GPT 6.0 Astra and I apply it to all existing projects in scope, a fresh thread in each affected project selects GPT 6.0 Astra without a separate project reset or composer change.
• The affected projects follow subsequent changes to the global model default, unless I deliberately configure a new project-specific override afterward.
• The action communicates that existing model overrides will be replaced and makes its machine scope and any skipped offline machines clear.
• Project settings show that affected projects now inherit the default, and the new-thread model selector agrees with that effective setting.
• Existing threads retain their model selections; the action controls the defaults for newly created threads.

Affected area

Settings → Projects, global and per-project model defaults, and model selection when creating a new thread in an existing project.

Non-goals

Changing the model in existing conversations, removing the ability to configure project-specific models, or changing unrelated project settings.

Alternatives considered

Resetting each project manually already provides a workaround, but it does not scale to many folders. Checking and changing the model in every new thread is easy to forget and leaves the stale project default in place.

Supporting context

Issue #6539 was closed after PR #9754 added machine-wide “All projects” defaults and per-project inherit/reset. This request builds on that functionality by adding a bulk way to make existing projects inherit the default.

Issue #8829 addressed automatically seeded project defaults. This request does not assume how the remaining overrides were created; it asks for a user-controlled way to replace them across existing projects.