chore: adopt one-worktree-per-session workflow

Sessions that change files now work in their own git worktree and merge back to
`develop`, instead of developing directly in the main checkout. Documents the
tool's actual behaviour rather than the assumed one: `EnterWorktree` prefixes the
branch as `worktree-<name>`, branches from the last commit (so uncommitted edits
do not carry over), and `ExitWorktree({action: "remove"})` deletes the branch
with the directory — hence the merge must happen before the cleanup.

Base ref is pinned to `head` in `.claude/settings.json` because `develop` runs
well ahead of `origin/main`, and the default `fresh` would branch from the stale
release trunk.

Also ignores `.claude/worktrees/`, which lives on disk per session and is never
committed.

.gitignore

@@ -17,6 +17,9 @@ desktop/dist-app/
 # local Claude Code settings (not shared)
 .claude/settings.local.json
 
+# per-session git worktrees (EnterWorktree) — live on disk, never committed
+.claude/worktrees/
+
 # logs / OS cruft
 *.log
 npm-debug.log*