Task: goto: add a task to an existing story

Extend compass goto so it can also start work on an existing story: branch + add a task to that story, instead of only ever creating a new story. Today goto always scaffolds a new story and fails if the slug already exists.

• Existing-story mode: compass goto --story <id-or-slug> --task-slug <slug> --task "<title>" [--description …] fetches main, creates a feature branch (feature/<task-slug>), adds a task to the resolved story (via add), sets the task #+branch:, and prints next steps.
• --story resolves by :ID: (UUID or unambiguous prefix) or by the story's folder slug, using doc_index.
• --story is mutually exclusive with the new-story flags (--slug=/–title=/=–description=); a clear error if both/neither are given.
• New-story mode is unchanged; --dry-run works for both.

Interface decision ("have a think"): keep the current new-story mode and add an existing-story mode keyed on --story:

• --story <id-or-slug> → existing-story mode. Resolve via doc_index: if the value is hex it matches a story :ID: (or prefix); otherwise it matches the story folder name (slug). Accepting both keeps it ergonomic (humans know slugs; tooling/links know UUIDs).
• Required with --story: --task-slug and --task (title); --description optional. Branch defaults to feature/<task-slug>.
• Mutually exclusive with --slug=/–title=/=–description= (new-story).

Steps:

1. Add --story / --task-slug to cmd_goto's argparse; validate the two modes are not mixed.
2. resolve_story(ident) → (story_dir, story_title) via doc_index.
3. In existing mode: branch from task-slug; skip story creation; one add task --parent-dir <story_dir>; set #+branch; next steps.
4. Dry-run + dogfood; update component overview + the goto recipe.

Verified via --dry-run for both modes (existing-story by folder slug and by :ID: prefix; new-story unchanged) and the three error paths (mixed modes, --story without task, unknown story). The execution path (git switch + add task --parent-dir) is the one already exercised by new-story goto and by add.

cmd_goto now has two modes. Existing-story mode: goto --story <id-or-slug> --task-slug <slug> --task "<title>" [--description …] resolves the story via resolve_story (by :ID: exact/prefix or folder slug), branches feature/<task-slug>, adds a task to the existing story (add), sets #+branch, and prints next steps. New-story mode is unchanged. The two modes are mutually exclusive with clear validation.