feat: expose account usage/quota information via SDK

Summary

Add the ability to query account-level usage and quota information through the Claude SDK, similar to what's shown at https://claude.ai/settings/usage

Use Case

When building applications on top of Claude Code (like Telegram voice agents), users want to see their usage stats including:

• Weekly token usage and remaining quota (for Max plan users)
• Rate limit status (requests/tokens per minute remaining)
• Context window usage

Current Situation

The SDK's ResultMessage provides per-request metrics:

• total_cost_usd - cost of the request
• usage - token counts for the request
• duration_api_ms - API duration

But there's no way to query:

• Account-level weekly usage/quota (Max plan)
• Rate limit headers (anthropic-ratelimit-requests-remaining, etc.)
• Overall context window state

Proposed Solution

Add a method to ClaudeSDKClient to query usage/quota:

async def get_usage(self) -> UsageInfo:
    """Get account usage and quota information."""
    ...

@dataclass
class UsageInfo:
    # Weekly usage (Max plan)
    weekly_tokens_used: int | None
    weekly_tokens_limit: int | None
    weekly_reset_time: datetime | None
    
    # Rate limits
    requests_remaining: int | None
    tokens_remaining: int | None
    rate_limit_reset: datetime | None

Alternatively, expose the rate limit headers from API responses.

Workarounds Attempted

• Checked CLI for usage command - not available
• Checked SDK response objects - only per-request data
• Web scraping claude.ai/settings/usage - not reliable/recommended