mship: external_symlinks config to handle Taskfiles that cd into sibling repos (../sibling pattern)

[atomikpanda]

Observed

Common metarepo pattern: a service Taskfile contains cd ../sibling-repo && task build (or similar relative-path navigation across repos). symlink_dirs only replicates a directory from the source checkout into the worktree. It does NOT make ../sibling-repo resolve to anything sensible from inside a worktree at .worktrees/feat/<slug>/<repo>/.

Today's only fixes are:

• Modify the inner repo's Taskfile to use absolute or env-based paths (often impossible — shared with non-mship users).
• Manually create symlinks after every spawn (loses with mship prune).

Proposal

Per-repo external_symlinks field in mothership.yaml:

repos:
  api:
    path: ./api
    external_symlinks:
      ../shared: /abs/path/to/shared       # absolute target
      ../auth: ${MSHIP_WORKSPACE}/auth     # workspace-relative via env
      ../infra: workspace://infra          # workspace-relative shorthand

On mship spawn:

• For each external_symlinks entry, create a symlink at <worktree>/<key> pointing at the resolved target.
• Refuse to overwrite existing files at the symlink path.
• Symlink absent? Drop it on mship close cleanup.

Why substrate-shaped

This is a path-management primitive — declare "this worktree needs these symlinks to function." It doesn't know anything about Taskfile syntax, language ecosystems, or methodology. Same shape as symlink_dirs, just for cross-repo paths instead of intra-repo dirs.

Acceptance

• mothership.yaml accepts external_symlinks: {<rel-path-in-worktree>: <target>} per repo.
• mship spawn creates the symlinks. mship close removes them.
• mship audit doesn't flag the symlinks as drift.
• Refresh strategy: see companion issue (mothership.yaml changes don't refresh existing worktrees).

Related

• Worktree layout breaks editable local deps and relative paths #87 — broader discussion of worktree layout breaking sibling paths. This issue is the focused, tactical proposal that doesn't require changing topology (whereas Worktree layout breaks editable local deps and relative paths #87 considers a hub-directory option as an alternative).
• Companion: refresh-existing-worktrees issue (mship symlinks refresh or extend bind refresh).