docs: jj workspaces live inside the repo under .workspaces

[runewright]

jj workspaces for parallel work live inside the repository under .workspaces/<name>.

Plan

Sibling-directory workspaces pollute the parent directory and break IDE workspace paths; the in-repo location keeps them discoverable and cleanable. The rule states the location and the cleanup command.

Changes

• Worktree rule rules/GitWorktrees.md: jj workspaces are created at .workspaces/<name> inside the repository, never as sibling directories.

Testing

• Push-gate prek checks green

Note

Update jj workspace guidance to place workspaces inside the repo under .workspaces/

Updates GitWorktrees.md to recommend jj workspace add .workspaces/<name> instead of jj workspace add ../<name> for jj-colocated repos.

• Adds a note that .workspaces/ should be added to .gitignore before creating the first workspace
• Adds rationale that sibling directories pollute the parent directory and can break IDE workspace paths

^{Macroscope summarized dbce9a4.}