refactor: split connection.go into focused, maintainable files (#291)

Fixes #273

Split the massive 3,587-line connection.go into 7 focused, maintainable files:

- connection.go (275 lines) - Main Connect() dispatcher, core types
- connection_browser.go (95 lines) - Browser launching, GUI detection
- connection_docker.go (145 lines) - Docker isolation setup and helpers
- connection_stdio.go (451 lines) - Stdio transport and command building
- connection_http.go (314 lines) - HTTP/SSE transports and auth strategies
- connection_oauth.go (2,189 lines) - All OAuth flows and error handling
- connection_lifecycle.go (243 lines) - Initialization and disconnect logic

Key achievements:
- 92% reduction in main file size (3,587 → 275 lines)
- 100% test pass rate - All tests pass
- No race conditions - go test -race passes
- No linter errors - Clean code quality
- No breaking changes - All public APIs preserved
- No circular dependencies - Clean dependency flow

Testing verified:
- All unit tests pass (100+ tests)
- Race detection clean
- Linter passes
- OAuth tests pass
- No circular dependencies

Author: Dumbris