fix(git): show changes across linked worktrees

The Git panel only ran git status against the session's own worktree,
so when DSH (or any coding agent) created a linked worktree via
'git worktree add' and modified files there, the sidebar's Git panel
stayed empty - no changed files, no diff, nothing to commit.

Enumerate all linked checkouts of the session repository and let the
user select which one the panel targets. Auto-select the dirty linked
worktree when the primary checkout is clean and exactly one linked
worktree has changes (the common DSH agent pattern). Poll status every
2s while the panel is visible so model-authored edits appear without
a manual refresh.

Security: resolveWorktree validates the client-supplied worktree path
against 'git worktree list' before using it as a git -C cwd. Uses -z
porcelain framing, which disables core.quotePath C-style quoting so
non-ASCII checkout paths round-trip losslessly.

src/client/sidebar.module.css

@@ -1959,6 +1959,20 @@ body[data-dsh-sidebar-dragging] .bottomPanel {
   padding: 0 8px 0 12px;
 }
 
+.gitWorktreeRow {
+  flex: none;
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 6px 8px 0 12px;
+}
+
+.gitWorktreeLabel {
+  flex: none;
+  color: var(--dsw-alias-label-tertiary);
+  font: var(--dsw-font-xxxs-11);
+}
+
 .gitBranchSelect {
   flex: 1;
   min-width: 0;