plan: single-file input shows 'delete' for unrelated pipelines

[jevy]

Problem

When running typestream plan with a single pipeline file, the output shows - delete for all pipelines not defined in that file:

$ typestream plan pipelines/categorization-failed-view.typestream.json

NAME                                   ACTION      CURRENT VERSION  NEW VERSION
bookkeeper-categorization-failed-view  + create                     1
bookkeeper-uncategorized-view          - delete     1

However, typestream apply on the same file is additive — it only creates/updates the pipeline in the file and leaves others untouched.

Expected behavior

plan and apply should have consistent semantics. Either:

1. Both additive (preferred): plan for a single file only shows what would happen to that pipeline, not unrelated ones. This matches how apply actually works today.
2. Both declarative: apply would also delete pipelines not in the input — but this is dangerous and would need a --prune flag or similar opt-in.

Option 1 seems safest and matches current apply behavior.

Workaround

Point plan at the full directory instead of a single file to get an accurate diff.