Add multi-currency cost display support

[chernistry]

What

Costs are currently hardcoded to USD ($X.XX). Add a configurable currency setting so users can see costs in EUR, GBP, JPY, etc. with a configurable exchange rate.

Why

Bernstein is used internationally. Seeing costs in your local currency makes budgeting more intuitive.

Where the current formatting lives

All cost formatting is in src/bernstein/cli/cost.py. Search for $ to find every place:

# Line 66-70 — main cost display
cons.print(f"  Single agent  [red]{single_bar}[/red]  [dim]${single_agent_cost:.4f}[/dim]")
cons.print(f"  Bernstein     [green]{bernstein_bar}[/green]  [bold green]${actual_cost:.4f}[/bold green]")

# Line 98-101 — summary
lines.append(f"   Cost:  ${actual_cost:.2f}")

# Line 418-419 — model breakdown
cost_str = f"${v['cost_usd']:.4f}"

# Line 439 — total
f"[bold green]${totals['cost_usd']:.4f}[/bold green]"

How to implement

Step 1: Add a format_cost() helper

Create a small helper in src/bernstein/core/cost.py (where MODEL_COSTS_PER_1M_TOKENS already lives):

# Currency symbols for display
CURRENCY_SYMBOLS: dict[str, str] = {
    "USD": "$",
    "EUR": "€",
    "GBP": "£",
    "JPY": "¥",
    "CAD": "CA$",
    "AUD": "A$",
    "CHF": "CHF ",
    "CNY": "¥",
    "KRW": "₩",
    "INR": "₹",
}


def format_cost(
    amount_usd: float,
    currency: str = "USD",
    exchange_rate: float = 1.0,
    precision: int = 4,
) -> str:
    """Format a USD cost amount in the target currency.

    Args:
        amount_usd: Cost in USD.
        currency: ISO 4217 currency code.
        exchange_rate: Multiplier to convert USD → target currency.
        precision: Decimal places.

    Returns:
        Formatted string like "$1.2345" or "€1.1234".
    """
    converted = amount_usd * exchange_rate
    symbol = CURRENCY_SYMBOLS.get(currency, currency + " ")
    return f"{symbol}{converted:.{precision}f}"

Step 2: Add config fields

In src/bernstein/core/models.py, add to OrchestratorConfig (around line 1024, near budget_usd):

currency: str = "USD"
currency_exchange_rate: float = 1.0

Step 3: Replace hardcoded $ formatting

In src/bernstein/cli/cost.py, replace all f"${amount:.Xf}" with format_cost(amount, cfg.currency, cfg.exchange_rate, precision=X).

Example before:

cons.print(f"  Bernstein     [green]{bar}[/green]  [bold green]${actual_cost:.4f}[/bold green]")

After:

from bernstein.core.cost import format_cost
cons.print(f"  Bernstein     [green]{bar}[/green]  [bold green]{format_cost(actual_cost, currency, rate)}[/bold green]")

Step 4: Add config to bernstein.yaml template

In templates/bernstein.yaml:

# Currency for cost display (default: USD)
# currency: EUR
# currency_exchange_rate: 0.92

Step 5: Write tests

# tests/unit/test_cost_format.py
from bernstein.core.cost import format_cost


def test_format_usd_default():
    assert format_cost(1.5) == "$1.5000"


def test_format_eur():
    assert format_cost(1.0, currency="EUR", exchange_rate=0.92) == "€0.9200"


def test_format_gbp_precision_2():
    assert format_cost(10.0, currency="GBP", exchange_rate=0.79, precision=2) == "£7.90"


def test_format_unknown_currency():
    assert format_cost(5.0, currency="BTC", exchange_rate=0.000015) == "BTC 0.0001"

Step 6: Run tests

uv run pytest tests/unit/test_cost_format.py -x -q

Acceptance criteria

• format_cost() function in src/bernstein/core/cost.py
• Config fields currency and currency_exchange_rate in OrchestratorConfig
• All $X.XX formatting in cost.py uses format_cost()
• Tests pass
• Pyright strict passes

[chernistry]

Implemented. See commits 932158a..d174c4d.