Add base_url configuration for Anthropic provider

[nsxdavid]

Summary

Adds support for configuring a custom base_url for the Anthropic provider, enabling use with proxy servers or alternative API-compatible endpoints.

Closes #51

Changes

• Added optional base_url field to Anthropic provider configuration
• Updated provider initialization to use custom base URL when specified
• Falls back to default Anthropic API endpoint when not configured

Example Configuration

[providers.anthropic.custom]
api_key = "your-api-key"
model = "claude-sonnet-4-20250514"
base_url = "https://your.custom.endpoint/v1"

Test plan

• Builds successfully
• Works with default Anthropic endpoint (no base_url specified)
• Works with custom base_url for alternative providers

[nsxdavid]

This makes it easy to pair G3 with something like my anthropic max plan router. Player and Coach arguing about things can otherwise get pretty costly. :)