chore(worktree): put worktrees under .claude and carry the overrides into them

.gitignore

@@ -53,3 +53,14 @@ app.*.map.json
 
 # Local sibling-package path wiring (dev only)
 pubspec_overrides.yaml
+
+# Worktrees Claude Code creates for parallel sessions and isolated subagents.
+# Only this subdirectory: the rest of `.claude/` is tracked, because the rules
+# under it are part of the repository.
+#
+# They sit inside the working tree rather than beside it. `git clean -xdf` leaves them
+# alone: git's nested-repository guard resolves a gitfile too, so it reports "Skipping
+# repository .claude/worktrees/<slug>" and the contents survive. `git clean -xdff` does
+# not skip them, and wipes a live worktree while `.git/worktrees/<slug>` survives,
+# leaving a registration that `git worktree prune` then has to clear.
+.claude/worktrees/

.worktreeinclude

@@ -0,0 +1,21 @@
+# Gitignored files copied into every worktree Claude Code creates, for `--worktree`,
+# for `EnterWorktree`, and for a subagent with `isolation: worktree`.
+#
+# A worktree is a fresh checkout, so `pubspec_overrides.yaml` is absent from it, and
+# its absence fails silently rather than loudly: the siblings resolve from pub.dev
+# instead of the working trees beside this one, `flutter pub get` succeeds, and the
+# suite then passes against the PUBLISHED packages while the diff under review is of
+# the local ones. An unreleased sibling API is where that bites.
+#
+# `.gitignore` syntax, and only files that match AND are gitignored are copied, so a
+# tracked file can never be duplicated by this list.
+#
+# The paths inside `pubspec_overrides.yaml` have to be ABSOLUTE for this to work.
+# Worktrees live under `.claude/worktrees/<slug>`, so a relative `../magic` resolves
+# to `.claude/worktrees/magic`, which does not exist, and version solving fails on
+# the first path dependency.
+#
+# NOT processed when a WorktreeCreate hook replaces the default git logic; such a
+# hook has to copy these itself.
+
+pubspec_overrides.yaml