US-1b.2implemented Add a new category after initial setup
As a user who discovers a new type of expense (e.g. `repairs:bike`) I want add a new category to `categories.yaml` without breaking existing categorised transactions so that I can evolve my category taxonomy over time.
Acceptance Criteria
- Adding a new leaf or branch to `categories.yaml` does not invalidate previously labelled receipts or classified transactions.
- The new category appears in the TUI labeller and the rule-based classifier.
No DAG diagram: Schema evolution concern — no data flows through layers; it modifies an existing config file in-place.
Demo video coming soon