US-4.5implemented Include opening balances from a starting journal
As a user starting bookkeeping mid-year with existing account balances I want provide a `start_journal_filepath` that contains my opening balances (e.g. 1000 EUR in checking) so that the `all-years.journal` reflects my correct total balances, not just the delta from imported CSVs.
Acceptance Criteria
- The starting journal is appended as an `include` directive in `all-years.journal`.
- `hledger bal` shows the correct total balances including the opening position.
No DAG diagram: The opening balance journal is referenced via start_journal nodes in other stories' paths (e.g. start_2024_1000eur); this story describes the requirement but its data is exercised by US-4.1 and others.
Demo video coming soon