feat: add configurable API timeout with higher default

[johnib]

Summary

Adds configurable API timeout functionality to prevent legitimate long-running operations from failing due to hardcoded timeouts.

Fixes #97

Changes

• New CLI option: --timeout (alias -t) to set API timeout in milliseconds
• Higher default: Increased from ~30s to 120000ms (2 minutes) for better reliability
• AbortController implementation: Proper timeout handling across all API calls
• Comprehensive coverage: Applies to both chat completions and GitHub token polling
• Documentation: Updated README with usage examples and tips

Usage Examples

# Use default 2-minute timeout
npx copilot-api start

# Set 3-minute timeout for complex operations
npx copilot-api start --timeout 180000

# Short alias also works
npx copilot-api start -t 300000

Technical Implementation

• Implements AbortController with setTimeout for clean cancellation
• Graceful error handling for timeout scenarios
• Backward compatible with existing functionality
• Added timeout parameter to state management
• Proper cleanup with finally blocks

Benefits

• ✅ Prevents legitimate long-running operations from failing
• ✅ Maintains protection against truly hung requests
• ✅ Particularly improves Claude Code experience with complex tasks
• ✅ User-configurable based on specific use cases
• ✅ Better out-of-the-box reliability with higher default

Testing

• ✅ Passes all existing tests
• ✅ Lint and typecheck validation
• ✅ Timeout functionality verified in development

This resolves timeout issues that users experience with complex prompts, large context windows, and multi-step reasoning operations that regularly exceed the previous timeout limits.