Task: Implement the goto command

Implement compass goto: one command that fetches main, creates a feature branch, scaffolds a linked story + task (codegen as a library), sets the task's #+branch:, and prints the remaining manual steps.

• compass goto --slug … --title … --description … [--tags …] [--task …] fetches main, git switch -c feature/<slug>, scaffolds the story (current sprint) + a linked task, and sets the task #+branch:.
• --dry-run prints the plan (branch / story / task / sprint) with no side effects; --base overrides the branch base.
• Generation goes through v2_doc_generate (shared _import_generator with add) — no duplicated generator logic.
• Prints next steps (wire into sprint table, set states, push/PR); does not silently edit the sprint table.

1. Share the lazy generator import between add and goto (_import_generator).
2. cmd_goto: argparse (slug/title/description/tags/task/base/dry-run); resolve current sprint; git fetch + git switch -c; two v2_doc_generate.main calls; _set_frontmatter_branch; next-steps.
3. Short-circuit goto in main() (own argparse); register for --help.

Dogfooded — though goto can't scaffold its own story (that already exists), it was verified by --dry-run and a throwaway real run (branch + story + task created, #+branch set), then cleaned up.