feat: worktree session UI (dialog checkbox, confirm delete, list marker)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -32,6 +32,27 @@ hqt
 | `r` | Rename session |
 | `s` | Stop session |
 
+## Worktree-isolated sessions
+
+The New Session dialog (`n`) has an **Isolate in worktree** checkbox. When the
+selected project is a git repository, checking it runs the session in a
+dedicated [git worktree](https://git-scm.com/docs/git-worktree) instead of the
+project root, so concurrent sessions don't step on each other's working tree.
+
+- The worktree lives at `<project>/.worktrees/<branch>` and is automatically
+  added to the repo's `.git/info/exclude` so it never shows up as untracked.
+- The branch is created off the current `HEAD`. If you leave the branch field
+  blank it defaults to a slugified form of the session nickname.
+- For non-git projects the checkbox is disabled.
+
+Deleting a worktree session (`d`) opens a confirmation dialog with an **Also
+remove worktree** option. As a safety check, if the worktree has uncommitted
+changes or unmerged commits a warning is shown and the removal box is left
+unchecked — checking it then forces removal and discards that work.
+
+Known limitation: submodules are not initialized in freshly created worktrees;
+run `git submodule update --init` inside the worktree if your project needs them.
+
 ## Architecture
 
 hqt uses a layered architecture: a Click CLI bootstraps into a Textual TUI, which drives service classes (ProjectService, SessionService) backed by SQLite via SQLAlchemy. Sessions are spawned as tmux windows through TmuxManager, with harness-specific configuration provided by pluggable HarnessConfigurator implementations discovered at runtime.