fix(#625): never remove a worktree that is, or holds, a Docker case workspace

The Docker rule was applied to the CASE folder and its common git dir
only. A Docker case can be linked to any host folder, including a managed
worktree (`<repo>/.claude/worktrees/<name>`), which is a natural setup for
running one of the parallel agents in a container. Removing that worktree
from the local case's dialog, or through Close Session's "Kill & remove
worktree", ran `git worktree remove`, which runs a git status INSIDE the
tree (container-written files, checkout filters on this machine) and then
deletes the whole tree, ignored files included, while the container still
mounts it. Reproduced on an isolated instance: DELETE answered 200 and the
Docker case's workspace was gone.

That contradicts the invariant the PR itself documents in CLAUDE.md ("git
never runs ... at or inside a Docker workspace"). removeCaseWorktree now
refuses (400) a present worktree that is, sits inside, or holds a Docker
case workspace, before any git runs; the DELETE route passes the Docker
roots it already reads for the case check. A worktree whose folder is gone
is unaffected (git only drops its record there). Unit and route tests
cover both directions and an unrelated Docker workspace; docs updated.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/wiki/Git-Worktrees.md

@@ -62,7 +62,9 @@ If a worktree's folder was deleted by hand, its row says **missing** and offers
 
 - **Docker cases and remote (SSH) cases.** A container can write to its workspace, and creating
   a worktree runs the repository's hooks and checkout filters on the Codeman machine, so git is
-  never run there. A remote case's repository is on another machine.
+  never run there. A remote case's repository is on another machine. For the same reason a
+  worktree that a Docker case uses as its workspace (or that holds one) is never removed from
+  here: remove the Docker case first.
 - A case folder that is not the top of a repository (a subfolder of one), a bare repository, or
   an empty repository with no commit yet.
 - Worktrees outside `.claude/worktrees/` (made by hand with `git worktree add`) are listed, but