Per-project default environment for new threads

[webmarkyn]

Area

apps/web, packages/client-runtime, packages/contracts

Problem or use case

I connect a remote environment and add the same repository on my local machine and on the remote. Project grouping merges the two checkouts into one sidebar project.

For some projects, I always want to run threads on the remote. Today I must pick the remote each time I start a new thread. There is no setting to make the remote the default for one project.

Current behavior in the source:

• buildProjectGroups picks the member on the primary (local) environment as the group representative (packages/client-runtime/src/state/projectGrouping.ts).
• The "New thread in…" palette uses that representative when no thread from the group is active (buildSidebarProjectPickerEntries in apps/web/src/sidebarProjectGrouping.ts).
• The legacy sidebar shows an environment menu for each new thread in a grouped project.
• defaultThreadEnvMode only selects local or worktree mode. It does not select the machine.

Current workarounds have costs:

• "Remove checkout" on the local copy deletes its threads.
• Turning off grouping shows each repository twice.

Proposed solution

Add a per-project preferred environment setting:

• Store it next to sidebarProjectGroupingOverrides, keyed by the logical project key. The value is an EnvironmentId.
• Add a "Default environment" select in project settings. Show it only when the group has members on more than one environment. The default value is "Primary environment", which keeps today's behavior.
• Use the setting when the app resolves the target member for a new thread: the sidebar button, Cmd+N with no active thread, the "New thread in…" palette, and the legacy sidebar menu (preselect it, or skip the menu).
• If the preferred environment is disconnected, fall back to the primary environment and show the environment in the composer.

The active-thread rule stays the same. A new thread from an open thread still uses that thread's environment.