Allow configurable copilotBaseURL in Copilot provider

[blinkagent]

Problem

The Copilot provider in provider/copilot.go uses a hardcoded default base URL (https://api.individual.githubcopilot.com). While config.Copilot has a BaseURL field and NewCopilot() respects it when provided, there's no way to set this via environment variables or config files.

This limits users who need to:

• Route requests through a proxy
• Use GitHub Copilot for Business/Enterprise endpoints
• Test against mock servers
• Comply with network policies

Proposed Solution

Add support for configuring the Copilot base URL via environment variable (e.g., BRIDGE_COPILOT_BASE_URL), following the existing pattern for BRIDGE_DUMP_DIR.

Acceptance Criteria

• Users can override the default Copilot base URL via env var
• Documentation updated
• Default behavior preserved when no override is specified

Additional Context

The config.Copilot struct already has the BaseURL field - this is purely a configuration wiring issue.

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Created on behalf of @matifali