refactor: implement context-aware network dialing

Modernize network dialing to use Go's context-aware patterns for better
cancellation support and connection reliability.

Changes:
- Update Go version from 1.24 to 1.25 in go.mod
- Replace ssh.Dial with net.Dialer.DialContext + ssh.NewClientConn
  for SSH client connections in serverconnection.go
- Add TCP KeepAlive (30s) for SSH connection health monitoring
- Implement context-aware dialing for SSH agent connections in ssh.go
- Improve error messages to distinguish dial vs SSH handshake failures
- Update AGENTS.md with integration test requirements

Benefits:
- Context cancellation now properly affects connection establishment
- TCP KeepAlive prevents silent connection failures
- Better integration with Go's cancellation patterns
- Improved reliability for distributed systems

All integration tests pass with race detection enabled.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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