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Paste-ready:

Title: Allow per-project default workspace type: branch instead of worktree

Body:

Please add a per-project (or global) setting to choose the default workspace type for new sessions in the GitHub Copilot app — specifically the ability to default to branch sessions (running in the project's existing local checkout) instead of worktree sessions.

Context: My local development server (MAMP Pro) serves a fixed DocumentRoot pointing at the project's main checkout. Worktree sessions run in a separate directory, so the served code silently diverges from the code under test — a mismatch that recently cost a debugging session because environmental staleness presented as application bugs. Custom instructions can request branch sessions, but the app offers no way to set this as a default, and a session's workspace type cannot be changed after creation.

Requested behavior: a project-level setting (e.g., in project configuration) to select "branch" or "worktree" as the default for new sessions, so tooling that serves the main checkout always serves the code being worked on.

No repo changes this turn — text only.

Start at the /chat workspace and session-creation flow, then trace how project configuration is currently loaded and how branch versus worktree sessions are selected. Done means a project-level or global default can choose branch or worktree for new sessions, with the selected default reflected in newly created sessions.