fix(files): list the git worktree an agent moved into (#765)

Claude Code's EnterWorktree moves the running process into
<repo>/.claude/worktrees/<name>, while Session.workingDir stays on the main
checkout. The Files panel kept walking workingDir, so it listed a tree the
agent had left.

paneCwd already follows the move, measured on Claude Code 2.1.318 in both
directions, so nothing new has to track the agent. It also follows every cd
in a shell, so a new SessionState.liveWorktree narrows it to a linked
worktree, read from the .git file without starting git.

Only a folder strictly below workingDir is ever an answer, and the listing
only starts there: returned paths stay relative to workingDir, so the
preview, raw and edit routes are unchanged and confinement does not widen.
A worktree outside the session folder stays unfollowed.

The panel asks with root=live, shows a strip naming the worktree with a
switch to the session folder, and follows the agent in and out while open.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.changeset/files-panel-live-worktree.md

@@ -0,0 +1,5 @@
+---
+"aicodeman": patch
+---
+
+Files panel: when a session's agent moves into a git worktree part way through (Claude Code's EnterWorktree), the panel now lists that worktree instead of the main checkout, follows the agent back out, and shows a strip that names the worktree with a button to switch to the session's folder. Only a worktree inside the session's folder is followed, and file access stays confined to that folder as before.