Document SlipJay workspaces and Land for users #44

Part of #21. From the Spec review of #21 (Spec 6).

Outcome

Users of a SlipJay project can find out from the user docs that each new thread gets its own SlipJay workspace, what the composer strip shows, and how Land works and reports.

Problems found

• docs/user says nothing about automatic workspaces, the strip, the landing state, Land, or Git actions being absent in Jujutsu and colocated repositories.
• The Settings row text still says "new worktree threads", although every thread in a SlipJay project now allocates a workspace.

Acceptance

• One concise section in the relevant docs/user guide, following the AGENTS.md documentation rules: what happens, how to start, and the unintuitive parts. For example, Land removes the landed workspace, and it needs a SlipJay runner location.
• The Settings text matches the behaviour.