Docs and Settings say new threads get a workspace except explicit project-checkout launches #53

Part of #21. From the final Spec review of #21 (Standards 3).

Outcome

The user docs and the Settings Workspaces text match #46: a new thread in a SlipJay project gets its own workspace, except an explicit project-checkout (root) launch.

Problem found

docs/user/source-control.md and the Settings row say "Every new thread ... gets its own SlipJay workspace". #46 keeps explicit root launches (scheduled "project checkout" tasks and MCP {type:"root"}) on the main checkout, and docs/user/project-settings.md says so.

Acceptance

• Both texts state the exception once, briefly, without implementation detail.