chore(makefile): add worktree-new/worktree-rm targets for isolated dev servers

Encode the dual-repo worktree SOP as make targets so no step is skipped:
worktree-new creates the outer worktree off main, inits its own user/
submodule, branches both repos, and starts an isolated Grav dev server on an
auto-picked free port (8090+) whose identity is persisted in a git-ignored
.worktree-env; worktree-rm tears it all down including the submodule deinit
that, when skipped by hand, leaves orphaned .worktrees/ dirs.

docker-compose.yml container_name + ports are parametrized as ${VAR:-default}
so the main checkout is byte-identical, and the 11 hardcoded intotheeast_grav
refs in local targets now use $(GRAV_CONTAINER). CLAUDE.md points at the
commands instead of the manual steps.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RDS6t8wcpbwKvvrxykVQ5K

.gitignore

@@ -2,6 +2,8 @@
 .env
 .env.prod
 .env.test
+# Per-worktree dev-server identity, written by `make worktree-new`
+.worktree-env
 
 # Grav CMS
 /user/